1) The auctions are a good idea and we need not be concerned that the people who purchase items from the shul are violating a prohibition of receiving benefit from their Mitzvah.

2) I assume that the question is that sometimes the shul sale works out cheaper than buying flowers, etc., in the store (because the workers in the shul sale are volunteers, the shul does not have overhead expenses, etc.). However

the scenario is different from that of the Gemara we are learning now in Daf Yomi. The Gemara prohibits only receiving wages in return for doing a Mitzvah. In contrast, a person who bought an item from the shul sale is not receiving direct wages for doing a Mitzvah, but rather is receiving (and only sometimes) an indirect benefit.

3) The shul should not overcharge. The source for this is the Mishneh Berurah (OC 153:45) who writes that if a shul sells movable items there certainly is a prohibiton of "Ona'ah" (overcharging). Therefore, one must not charge one sixth more than the market price (see Bava Metzia 49b). Even though the Shulchan Aruch (CM 227:29) states that the prohibition of Ona'ah does not apply to consecrated items, the SM'A there (#49) notes that property belonging to a shul in our times are considered as non-holy items with respect to this Halachah.

4) However there might be a certain leniency here. This may be with respect to what is stated in the Shulchan Aruch (CM 327:6) that there is a doubt in the Halachah about whether it is permitted in the first place to overcharge by less than a sixth. The Rosh writes that a G-d-fearing person should be stringent about this. See the Chasam Sofer on the Shulchan Aruch, who says that such a person should be careful not to overcharge his customer even by less than a sixth. In other words, there is a doubt in the Halacha about whether one is permitted l'Chatchilah to overcharge by less than a sixth, and it is praiseworthy to be Machmir in this matter.

Nevertheless, I suggest that it is logical that a shul holding a sale need not be stringent in this matter and, therefore, if the market value of the item is $6 the shul may charge up to $6.99 for it. This is because we may assume that when people buy from a shul sale they are aware that they are also performing a Mitzvah of giving Tzedakah by doing so, and consequently they are Mochel and are not particular if they are overpaying a little.

5) After writing the above, I posed your question to a big Talmid Chacham who gave an explanation which, I think, is similar to my answer above (in #2), as to why buying from shul auctions is not considered as though one is receiving benefit from a Mitzvah, but his sources are stronger.

a) The basic idea is that saving money is not equivalent to receiving benefit. A source for this is the Mishnah earlier (33a) which teaches that if Reuven made a vow that Shimon must not derive benefit from him, Reuven is still allowed to pay off the debt of Shimon. The Gemara there says that the reason is because paying off somebody's debt is comparable to chasing a lion off his field. Reuven is merely preventing Shimon from losing out by having to pay off his debt; preventing this loss is not the same as Shimon receiving direct pleasure from Reuven. It is only direct benefit which Shimon is forbidden to gain from Reuven.

Similarly, it is only direct wages that one is not allowed to receive in return for doing Mitzvos, while merely saving money (because if one would not have bought the flowers in the shul sale, one would have had to pay more in the store) is not considered as receiving payment for performing Mitzvos.

2) The Talmid Chacham I asked also cited Tosfos in Chulin 131a, DH Sha'ani. The Gemara there teaches that if the king confiscates Reuven's wheat because he owes the king money, Reuven still must give Terumah to the Kohen from this wheat; he cannot argue that the wheat no longer belongs to him and thus he is exempt from tithes. The question is, what is the difference between this case and the Gemara in Chulin 130b, that if somebody eats up the presents that he should have given to the Kohen before he actually gave them, he is exempt from payment? Why do we not say that just as the person whose wheat was confiscated from him by the king must still separate tithes from this wheat, because the wheat brought him benefit as it paid back his debt to the king, so, too, the person who ate the wheat before he had time to give it to the Kohen is also considered to have benefited from the wheat because otherwise he would have to pay out money to purchase different food?

Tosfos answers this question with a big Chidush. He asserts that eating the wheat is not considered benefit, because if he would not have eaten the wheat, he might have fasted. In contrast, when the king took his wheat, this was a clear and definite benefit for the owner, because the king would have certainly taken other property in return for his debt if he would have not been able to take this wheat.

We learn from Tosfos that merely saving money on food bills is not considered by Halachah as Hana'ah, benefit. Similarly, the person who bought cheap flowers at the shul sale is not considered as gaining benefited from the Mitzvah of helping the shul, because if he would not have bought flowers in the shul sale, he may not have bought flowers at all.

Kol Tuv,

Dovid Bloom

This is not a Psak Halachah