Gemara

(Mishnah): One who sells garments [with Sha'atnez] may sell normally, as long as he does not intend for [protection from] the sun or rain.

Tzenu'im (pious people who are careful to avoid suspicion) hang such garments on a stick in back of them.

Kil'ayim (Mishnah 9:6): People who sew garments [of Kil'ayim] sew normally, as long as they do not intend in the sun due to the sun, or in the rain due to the rain. The Tzenu'im sew on the ground.

(Mishnah 2): One may not wear Shatnez, even on top of 10 layers of clothing, to evade the tax.

Bava Kama 113a: This is unlike R. Akiva;

(Beraisa): One may not evade the tax;

R. Shimon says, R. Akiva permits (wearing Shatnez) to evade the tax.

Question: Granted, the first Tana forbids Davar she'Eino Miskaven (something one does not intend for), and R. Akiva permits;

However, all should forbid to evade the tax!

(Shmuel): The law of the kingdom is binding.

Answers: The Beraisa discusses a tax collector who takes as much as he wants, or the king did not appoint him.

Menachos 41a - Suggestion: The Isur of Kilayim does not apply to a garment too small for a child.

Rejection (Mishnah): The Isur of Kilayim does not depend on whether one would wear the garment on occasions.

Yevamos 4a: Had it said only "It will not come upon you", one would think that even a clothing salesman may not put on Sha'atnez (to display it);

"Do not wear" teaches that it is forbidden only to wear it like clothing, i.e. when he benefits from it.

Rishonim

Rambam (Hilchos Kil'ayim 10:16): People who sew garments may sew normally, as long as they do not intend for the sun or rain. Tzenu'im sew on the ground. Similarly, people may sell normally, as long as they do not intend that the Kil'ayim on his shoulder save him from the sun or for warmth in winter. Tzenu'im put it on a pole in back of them.

Rambam (18): One may not wear Kil'ayim haphazardly, even on top of 10 layers of clothing and it does not benefit him at all, even to evade the tax. If he wore in such a case, he is lashed.

Rosh (Hilchos Kil'ai Begadim 9, after Hilchos Nidah): There is no Heter to wear Kil'ayim haphazardly. This is called wearing, and it is forbidden (Menachos 41a). The Gemara connotes that we discuss wearing a child's garment. One may not wear Kil'ayim even on top of 10 layers of clothing, even to evade the tax. They do not collect taxes on clothing that one is wearing. This Tana forbids Davar she'Eino Miskaven. He argues with the Tana of Mishnah 5, who permits sellers to sell normally. The Halachah follows that Mishnah, for we rule like R. Shimon, who permits Davar she'Eino Miskaven.

Poskim

Shulchan Aruch (YD 301:5): One may not wear Kil'ayim haphazardly, even on top of 10 layers of clothing and it does not benefit him at all, even to evade the tax.

Shulchan Aruch (6): People who sew garments may sew normally, as long as they do not intend in the sun for the sun or in the rain for the rain. Tzenu'im sew on the ground. Similarly, people may sell normally, as long as they do not intend that the Kil'ayim on his shoulder save him from the sun or for warmth in winter. Tzenu'im put it on a pole in back of them.

Beis Yosef (DH u'Mah she'Chasav): The Rambam rules like the Mishnah that forbids wearing Kil'ayim to evade the tax, and also like the Mishnah that permits people to sell normally. I say that he holds that all agree to the first Mishnah. Even R. Shimon, who permits Davar she'Eino Miskaven, forbids, for he truly wears it. He is lashed whether or not he intends. However, people who sell or sew Kil'ayim do not really wear them. They put them on themselves, unlike wearing. This is permitted if they do not intend. He argues with the Tur's explanation that they wear them to show the size. He holds like his other Perush, that they put it on themselves when carrying them to [where they will] sell them. This is why the Rambam (Halachah 17) forbids winding ropes of Kil'ayim together n his hand, for this is like wearing.

Bach (5): The Tur teaches that it is permitted not only to evade the tax, which is Ones, for the tax collector wants to steal from him, and the only way to save himself is through wearing Kil'ayim. It is a Pesik Reishei d'Lo Nicha Lei (he is not pleased with the result). Surely this is permitted. Rather, even if he puts Kil'ayim on himself or wears it in order to sell it, and he could sell it without this, it is permitted due to Davar she'Eino Miskaven. This is not called a Pesik Reishei when he wears it only to show the size, for perhaps his body will not benefit from it. Similarly, people who sew Kil'ayim do not Vadai benefit. This is unlike wrapping ropes, which is a Pesik Reishei Reishei d'Nicha Lei. The Rambam rules like the Mishnah that forbids wearing to evade the tax, for it is a Pesik Reishei. He says that one is lashed mid'Oraisa. He permits sellers. The custom is to be lenient in every case, like I explained.

Taz (7): It is permitted even though while sewing, the garment is on their knees, because it is Davar she'Eino Miskaven. This contradicts the Shulchan Aruch's Isur to wear Kil'ayim to evade the tax! The Gemara says that R. Shimon permits. The Shulchan Aruch is like the Rambam. The Beis Yosef forbids when he truly wears the garment, even without intent. I do not understand his distinction. The Torah forbids even partial wearing, i.e. wrapping around his hand. Why should we be more stringent, and say that one is lashed mid'Oraisa, when he wears to evade the tax, than wearing on part if the body? Also, the Tur permits sellers even if they wear in order to show the size. The Beis Yosef says that the Rambam disagrees. What do they argue about? The Bach says that the Rambam forbids in order to evade the tax, for it is a Pesik Reishei. He overlooked the Sugya in Bava Kama. It explicitly says that R. Shimon permits. I say that the Rambam learns from Perek 9 of Kil'ayim. The Tana taught the Isur to evade the tax, and later the Heter of selling. The Rambam did not want to say that Mishnayos within a Perek argue with each other! Rather, the Tana distinguishes. One gets Hana'ah through evading the tax, for if he did not wear it, he would need to pay the tax. This is like other Hana'ah of wearing that the Torah forbids. This is like the Shulchan Aruch (Sa'if 12) forbids using Kil'ayim to hold a hot egg and be saved from its heat. Sellers are permitted, for the sale does not depends on this, just he wears it to lighten his burden or to show the size. He could do so in another way. Since the wearing is not essential, if he does not intend for Hana'ah, it is permitted.

Taz (8): Regarding people who sew, the Rambam wrote like the Shulchan Aruch. Regarding sellers, he wrote that they may not intend to be saved from the sun, or for warmth in winter. Why didn't he say so about people who sew? Why didn't he mention sellers being saved from the rain? It seems that he discusses the normal case. Surely, he forbids also intent to be saved from rain. Sellers carry Kil'ayim on their shoulders. This would not save them from rain. His entire body will get dirty from rain, and only his shoulders are spared. He can be saved from sun, for he holds it up towards the sun, and makes shade for his entire body. Either way it is forbidden. This is why the Tur and Shulchan Aruch omitted this.

Prishah (10): The Maharshal asked about one who does not intend for warmth, rather, so his clothes not get ruined. This requires investigation. Why is this different than using Kil'ayim to take a hot egg, to be saved from the heat? Why is rain different? I compare this to a garment [of Kil'ayim] put on one who takes a haircut. If it has does not have something to hold the head it is permitted, even though it protects the clothes from getting dirty with hair. The same applies to saving his garments from rain.

Taz (8): I cannot believe that the Prishah wrote this. We learn oppositely from garments for haircut! The Beis Yosef brought from the Rosh that the Yerushalmi says that if he intends that it be a garment, i.e. it has [a hood] to hold the head [it is forbidden]. It is forbidden even though it is only to save from filth! The same applies to saving from filth of rain. See Sa'if 10.

Shach (8): The Beis Yosef says that this is not worn like a garment, but in Sa'if 5, it is. I ask that in any case, it is Davar she'Eino Miskaven! I say that in Sa'if 5, since he intends to wear it like a garment, it is Davar she'Miskaven. Here, he intends only that people see it and buy. The Rambam explains the Mishnayos nicely. It is difficult to say that Mishnayos in one Perek argue with each other. One opinion in Sa'if 6 permits even wearing without intent. However, in Sa'if 8 all forbid a saddle to take out manure on it. He does not intend [to wear it]! The Rosh, Tur and R. Yerucham explicitly say so. In Sa'if 8, the Levush forbids even though he intends only to save from filth, even Ha'alah (putting it on him without wearing it). This is difficult even according to my Perush of the Rambam. It seems that taking out manure is called intent. He takes the saddle in order to save his clothes from filth. This is like taking a hot egg in a garment of Kil'ayim (Sa'if 12), unlike the Levush, who considers it Davar she'Eino Miskaven.

Pischei Teshuvah (5): Merkeves ha'Mishneh (Hilchos Shabbos 1:1, p.14a-14b in standard editions) argues with the Taz and Shach. He says that surely, Davar she'Eino Miskaven is permitted, even though he surely does an Isur. If one's coins scattered in front of idolatry, it is forbidden [to bend down to gather them] only due to Mar'is ha'Ayin, even though he bows. This is the ultimate Pesik Reishei! Rather, something is not considered a Pesik Reishei if (this should say unless - PF) he intends for Heter, and the only way to do it is through an Isur, like when he wants to evade the tax and he cannot wear it without getting warmth. Therefore, intent to wear is like intent for warmth; it is a Pesik Reishei. The same applies to putting a saddle on his shoulder to carry a burden. Those who sell and sew garments are different. They have no intent to put the Kil'ayim on themselves, only to carry and bring it. This is not considered a Pesik Reishei, even though it is on his shoulder, since he could take it in his hand or on a stick without getting warm. To sew, the garment must (this should say need not be - PF) on his knees. Therefore, it is considered Davar she'Eino Miskaven, like coins that scattered in front of idolatry. It is Davar she'Eino Miskaven, since he could gather them without bowing. However, there we forbid due to Mar'is ha'Ayin. There is no Mar'is ha'Ayin regarding those who sell or sew garments, for all know that they engage in their work.

Rema: Some permit even to wear Kil'ayim, as long as he does not intend for Hana'ah, e.g. he wears it to evade the tax or to show its size if he wants to sell it, and similar matters.

Beis Yosef (DH ha'Lovesh): The Rosh and Semag rule like R. Shimon, who permits Davar she'Eino Miskaven.