(When R. Akiva said in the Mishnah that he may take (seeds) from the granary and sow and it is exempt from Maaseros until smoothed) it is like the shops of Beis Chanan in the Baraisa...

(Baraisa): Why were the shops of Bnei Chanan destroyed three years before the destruction of the Temple? Because they would exempt their produce from Terumos and Maaseros, as the Torah says (Devarim 14:22), "You shall surely tithe" to exclude one who buys produce and (verse 23), "and you shall eat" to exclude one who sells produce. (The Chachamim decreed however, that Terumos and Maaseros must be separated and R. Akiva only made the decree after smoothing; but the Bnei Chanan ignored even R. Akiva's obligation and did not separate at all.)

(R. Yochanan): (The implication of the Mishnah (above daf 15 (c)) was that if a Kohen or Levi bought produce from the granary after it was smoothed, they must give the Terumos and Maaseros to others.) The Chachamim penalized him so that he would not become accustomed to buying produce from the granary and wine press and making other Kohanim lose out.

(R. Yehoshua ben Levi): A Kohen who is selling his own animal - for the first week he is exempt from giving the shoulder, cheeks and stomach to another Kohen.

(R. Yosi): I went to the south and (also) heard R. Chanan, father of R. Shimon say from R. Yehoshua ben Levi that he is exempt for one week.

R. Yudan explained that the penalty after the first week is comparable to the Mishnah that required the Kohen and Levi to give their Terumos and Maaseros to others, in order not to cause others a loss.

(R. Yosef to R. Yudan): If their reason is this, then it should even apply to the first week?

A Kohen who opened a stall to sell meat - R. Yudan said that he is exempt for one week from giving away the shoulders, cheeks and stomachs. R. Yosi said that he is not exempt, even the first week.

According to R. Yosi, what is the difference between selling an animal and opening a stall? One who opens a stall could be deceptive. (Since he travels from town to town, when he comes to a new place he could claim that it is his first week.) One who sells his own meat cannot be deceptive (because he remains in his town where everyone recognizes him).

Baraisa: B'Ribi and R. Yehuda HaNasi agreed - that if one buys attached produce from a gentile, it is obligated in Maaseros; if one buys attached produce from a Yisrael, he takes on the role of the seller (and even if the buyer is poor, he may not keep the Maaser Ani for himself); if one buys detached produce from a gentile, it is exempt from Terumos and Maaseros.

When do they disagree? If one buys detached produce from a Yisrael in a year of Maaser Ani - R. Yehuda HaNasi says that whether the buyer is rich or poor, he must give away the Maaser Ani; B'Ribi says that if he is poor, he may keep the Maaser Ani.

Question: What is R. Yehuda HaNasi's reasoning? Just as a person may not keep his own Leket, Shichechah and Peah (even if he is poor), so too a person may not keep his Maaser Ani.

Question: What is B'Ribi's reasoning? The produce is not prohibited (as Tevel) before the Leket Shichechah and Peah is given, but it became obligated in Maaser Ani whilst still in the seller's hands (and is considered the seller's Maaser Ani) - therefore, the buyer may keep it.

R. Yehuda HaNasi's reasoning is logical, because there should be a penalty (to prevent poor people from constantly buying produce and causing the other poor people to lose out on the Maaser Ani). B'Ribi's reasoning (that there should be no penalty) is logical because it is not common for a poor man to be able to buy produce and cause others a loss. R. Yehuda HaNasi says that it is common that he borrows money in order to buy produce and B'Ribi says that it is not common.