Demai, Chapter 7, Mishnah 7. The Mishnah deals with a person who buys produce from a homeowner who is an am ha'aretz - an ordinary person who owns fields of his own - and with the question of when one may separate maaser from one batch of produce on behalf of another.
The words of the Mishnah and their explanation:
"Halokeiach mi'baal habayis" - one who buys produce from a homeowner, from an am ha'aretz who owns fields of his own.
"V'chazar v'lakach mimenu shniyah" - and he went back and bought from him a second time.
"Me'aser mi'zeh al zeh" - he may separate maaser from one batch of produce on behalf of the second batch that he bought from him.
"Afilu mi'shtei kupos" - even if the produce came from two different baskets.
"Va'afilu mi'shtei ayaros" - and even if the purchases were made in two different towns.
The reason for this: as long as we know that the produce comes from the fields of that same homeowner - even if he owns fields in two different towns - it all comes from a single source. Either he tithed all of it or he tithed none of it, and since he is an am ha'aretz we are stringent and assume that he did not separate maaser. Therefore the buyer may separate from one batch on behalf of everything, since it all has the same status.
Two general conditions in separating maaser:
The produce must be of the same species. One does not separate maaser from one type of grain on behalf of another, such as wheat for barley or barley for wheat - and this has nothing to do with the laws of demai.
The produce must all be from the same year. One does not separate from one year's crop on behalf of another year's, though this too is not a matter of demai.
"Baal habayis shehayah mocher yarak ba'shuk" - a homeowner who was selling vegetables in the marketplace:
"Bizman shemevi'in lo mi'ginosav - me'aser me'echad al hakol" - when they bring him produce from his own gardens, the buyer may separate maaser from one batch on behalf of all the rest of that same species, since it is all his.
"Mi'ginos acheiros - me'aser mikol echad v'echad" - if it is his practice that they bring him produce from other people's gardens, one must separate maaser from each purchase on its own, because each purchase may come from a different source, and we do not know that all the sources are alike as to whether maaser was separated from them or not.
The Yerushalmi notes that even when they bring him produce from other gardens, if the am ha'aretz says that the produce is untithed - that no maaser was separated from this crop - he may be believed, and therefore one may separate maaser from all of it on the assumption that no maaser was separated from any of it.
In summary: This Mishnah teaches that one who buys from a homeowner who is an am ha'aretz may separate maaser from one batch on behalf of the other - even from two baskets and even from two towns - because it all comes from a single source and has the same status, provided that the produce is of the same species and from the same year. In the case of one selling vegetables in the marketplace: if the produce is from his own gardens, he separates from one batch on behalf of all of it, and if it is from other gardens, he separates from each purchase individually, unless the am ha'aretz himself testified that the produce is untithed.