(Mishnah): If one takes wheat to a Cusi or Am HaAretz miller (when he goes to pick up his flour) he can assume that it's already tithed and it's not from Sheviis (and there's no concern that his produce was switched for another's). But if he took it to a gentile miller, he must treat it as Demai.

If one deposits produce with a Cusi or an Am HaAretz, he can assume that it's already tithed and it's not from Sheviis. But if he deposited it with a gentile, it is treated as the produce of a gentile (and is Rabbinically obligated in Maaseros). R. Shimon says that it is Demai (as it might have been switched).

(Gemara) (R. Chiya citing R. Yochanan): If a Chaver's box of grain was switched in a (Jewish) miller's possession, if an Am HaAretz was known to mill there on the same day (it is therefore doubtful whether or not the miller exchanged his grains with the Chaver's), he must be concerned (for this); if not, he does not need to be concerned.

Question: Shouldn't he be concerned (even if the miller occasionally mills there)? How is it different to the case of the caravan of travellers? Wasn't it taught that if an Arab merchant was found to be selling non-Kasher food on one day, that day becomes a proof for all of the other days (meaning that for any given piece of food, they must assume that perhaps it is the non-Kasher one)...?

Answer: The Arab merchant is different as he definitely supplied the food (at some time); but here, the Am HaAretz might not have been milling on that day.

(R. Taifa Tzamukah citing R. Abahu): Cusim are believed over a deposit (and there is no concern that the produce was switched).

Question: Isn't this explicitly taught in our Mishnah about a Cusi, that one can assume that it's already tithed?

Answer: The Mishnah referred to before the Cusim were suspected of idolatry. R. Taifa came to teach that even after that, they were believed that it was not switched.

Question: Is a Cusi believed to say that he took it and replaced it with other tithed produce?

Answer: If you believe him that he took it, believe him that he replaced it with tithed produce; if you don't believe him that he replaced it, don't believe him that he took it (so it's the same produce as he originally brought)!

Rebuttal: No - he's believed that he took them but he's not believed that he replaced them with tithed produce.

Question (R. Yona): What is the case? If he said that he grew them, even if he is an Am HaAretz he is not believed. If he said that Ploni tithed them for me, even a Cusi would be believed...?

Answer (R. Ba): It follows the opinion that a Cusi is like a gentile - as there is a dispute - Rebbi says that they are like gentiles; Rabban Shimon ben Gamliel says that they are like Jews in all matters.

Question: In the first clause, the Mishnah said that if he took wheat to a gentile miller, it is Demai; but in the latter clause it said that it's like the gentile's produce?

Answer: The latter clause referred to when he deposited a box of produce and there is a concern that the gentile switched the box. The first clause referred to when he deposited produce - there, there's no concern that he switched them.