Gemara

Mishnah (Demai 2:4): Chachamim obligated a baker to separate only Terumas Ma'aser and Chalah (from Demai).

Question: We understand why he is exempt from Terumah Gedolah - Yochanan Kohen Gadol sent messengers throughout Eretz Yisrael and found that many people separated only Terumah Gedolah (and decreed about Demai);

We also understand why he is exempt from Ma'aser Rishon and Ma'aser Oni - ha'Motzi mi'Chavero Alav ha'Re'ayah;

But he should separate Ma'aser Sheni and eat it in Yerushalayim (he does not lose it)!

Answer (Ula): Government officials used to beat bakers and tell them to sell cheaply, therefore Chachamim were lenient.

Mishnah (Demai 5:1): One who buys from a baker separates Chalah and Terumas Ma'aser...

Question: Mishnah 2:4 obligates the baker to separate Terumas Ma'aser!

Answer #1 (Yerushalmi Demai 8b): It depends on whether or not it is Tahor (several Peirushim of this answer are given below).

Answer #2 (ibid.): It depends on whether he bought a lot or a little.

Demai (2:4): One who sells large quantities may sell Demai (and the buyer must tithe it), but a grocer may not.

Tosfos (DH Tanan, citing the Yerushalmi): We are stringent lest children (who buy from grocers and are wont to eat without tithing) eat Tevel, or because those who sell small quantities make large profit.

Rishonim

Rambam (Hilchos Ma'aser 9:12): Chachamim did not obligate a baker to separate Ma'aser Sheni from Demai, only Terumas Ma'aser, in order that it will be separated b'Taharah with Chalah.

The Rambam rules like Answer #1 above, for it is R. Yochanan's opinion (Yerushalmi).

Perush #1 (Radvaz): If the baker bought Tahor produce, he must separate Terumas Ma'aser b'Taharah so the Kohen will be able to eat it. If he bought Tamei produce he is exempt. This is why the Rambam mentioned separating Terumas Ma'aser b'Taharah.

Perush #2 (Ri Korkus): Mishnah 2:4 obligates a Chaver baker to separate Terumas Ma'aser; Mishnah 5:1 obligates one who buys from an Am ha'Aretz baker to separate Terumas Ma'aser.

Perush #3 Ridvaz (Demai 10a DH Hacha): Kneading is Tovel (forbids eating before separating Chalah), but l'Chatchilah one may wait to separate Chalah until it is baked. However, Chachamim require to separate Chalah from a Tahor dough immediately after putting water on the flour, lest it become Tamei. The baker must separate Terumas Ma'aser, for this must precede Chalah. He need not separate Ma'aser Sheni, for it need not precede Chalah.

Perush #4 (Tosfos, ibid.): Mishnah 2:4 discusses a baker who works b'Taharah, i.e. a Chaver (he is also a Chaver regarding Ma'aser). We must say that he sells to a Chaver, for the Mishnah does not require the baker to separate Ma'aser Sheni; one may not rely on an Am ha'Aretz to tithe even Vadai Tevel, all the more we do not give to him Demai lest he not tithe it! Chachamim allow bakers to rely on a Chaver buyer to separate Ma'aser Sheni, for bakers are forced to sell cheaply.

Perush #6 (Tosfos, ibid.): Both Mishnayos discuss buying from an Am ha'Aretz baker. If an Am ha'Aretz toiled to hire a Chaver to knead b'Taharah, presumably he also tithed.

Rambam (ibid.) The buyer separates Ma'aser Sheni.

Rashi (DH Lo) says that Chachamim relied on the majority of Amei ha'Aretz who take Ma'aser. This connotes that even the buyer need not separate Ma'aser Sheni.

Tosfos ha'Rosh (DH ha'Nachtomin): Even though the baker need not give Ma'aser Rishon or Ma'aser Oni, he must separate them, for they are Tovel. We do not infer from the suggestion 'he should separate Ma'aser Sheni and eat it in Yerushalayim' that in truth he need not separate it. We rely on the majority of Amei ha'Aretz who take Ma'aser regarding the Lav to eat Ma'aser Sheni outside Yerushalayim, but not regarding eating Tevel, which is punishable by Misah biYedei Shomayim. Alternatively, since it would be improper to call something Ma'aser Sheni and eat it outside Yerushalayim, one may rely totally on the majority and not separate Ma'aser Sheni at all.

Note: The Tosfos ha'Rosh does not explain why we do not rely on the majority regarding Terumas Ma'aser. Presumably we require separation of Terumas Ma'aser because it is only 1%, whereas Ma'aser Sheni is 10%. Even though the baker may eat Ma'aser Sheni, since he sells a large quantity it might be much more than all he needs to eat in Yerushalayim. Also, perhaps we are more stringent about Terumas Ma'aser because one who eats Tevel to Terumas Ma'aser is Chayav Misah and also steals from Kohanim.

Question: Why didn't Chachamim obligate bakers to redeem the Ma'aser Sheni onto a Perutah?

Answer #1 (brought in Tosfos ha'Rosh 9a DH Lo): Some say that one may redeem Hekdesh onto a Perutah, but not Ma'aser Sheni. They bring proofs from Gemaros about scheming to redeem Ma'aser Sheni cheaply; if it could be redeemed on a Perutah, why would people scheme? The proofs can be challenged.

Rejection (Ritva DH ha'Nachtomin): Ma'aser Sheni is not more stringent than Hekdesh, which may be redeemed onto a Perutah! Also, we learn Kerem Revai from Ma'aser Sheni, and nowadays we redeem Kerem Revai on a Perutah!

Answer #2 (Ritva, ibid.): Chachamim totally exempted bakers from eating the Ma'aser Sheni in Yerushalayim, and even from redeeming it so there will not be any qualms, and lest they not bother even to redeem on a Perutah.

Rambam (ibid.). This refers to one who sells in or at the opening to his store. However, if he sells to a retailer or in a store next to a retailer, he must separate also Ma'aser Sheni.

Source (Minchas Bikurim Demai 3:13): The Tosefta (Demai 3:13) obligates one who sells to a retailer or in a store next to a retailer, because the officials are unaware that he sells, so he is not pressured to sell cheaply.