Demai, Chapter 5, Mishnah 8: One who buys tevel from two places. In this mishnah we move from the discussion of demai to the discussion of tevel - produce from which terumah and tithes were certainly not separated.
"HaLokeach tevel mishnei mekomos - me'aser mizeh al zeh": - One who buys tevel from two places may tithe from one for the other.
Someone who buys tevel from two different places may tithe from one batch for the other. When both sellers state explicitly that the produce is tevel and that none of the tithes have been separated from it, they are believed, and both quantities are considered obligated in tithes; therefore one may tithe from one for the other.
"Af al pi she'amru: ein adam rashai limkor tevel ela l'tzorech": - Even though they said: a person may not sell tevel except when there is a need.
The mishnah notes that the Sages ruled that a person may not sell tevel unless there is a need for it. The reason: there is a concern that the seller will forget to inform the buyer of the status of the produce, and the buyer will stumble into the prohibition of eating tevel, which may not be eaten. Therefore one may not sell tevel except when there is a need.
What is that "need"?
For example: a chaver whose properly tithed produce became mixed with tevel cannot separate from the mixture itself, since he might separate from the produce that was already exempt from tithing. He is therefore required to buy tevel and separate from that tevel for the majority of the tevel in the mixture.
Even in this situation, tevel may only be sold to a chaver, who is trustworthy in matters of terumah and tithes. And what should an am ha'aretz do if such a case comes his way? He should approach a chaver, who will buy the tevel for him and separate on his behalf, as is explained in the Yerushalmi.
It emerges that our mishnah deals with someone who sold tevel for a permitted need, or after the fact - once the tevel has already been sold even though it should not have been sold. In each of these situations, when two different sellers testify that both quantities are tevel, they are believed, and one may tithe from one for the other.
In summary: In this mishnah we moved from demai to tevel, and we learned that one who buys tevel from two places tithes from one for the other, since the sellers are believed when they report that the produce is tevel. We also examined the restriction "a person may not sell tevel except when there is a need" - out of concern that he will forget to inform the buyer - and what that "need" is: separating for a mixture of tithed produce and tevel, and even then only in a sale to a chaver, who is trustworthy.