Demai, Chapter Five, Mishnah Seven
1) One who buys from a field owner, and then buys from him again, he may give tithes from the one [purchase] for the other, even when [the purchases come] from two baskets and even from two towns.
2) A field owner who was selling vegetables in the market: when he brings them from his garden, he may tithe from one for all;
3) But [if he brings them] from other gardens, he must tithe each lot separately.
Section one: If one buys directly from an am haaretz field owner he can tithe from one purchase for the other, under the assumption that the field owner may not have tithed any of his produce, and therefore the purchaser wont separate tithes for non-tithed produce from other produce that has already been tithed. If the am haaretz did separate tithes, then there also will not be a problem, because he would have separated tithes from all of his produce. Therefore, the purchaser can separate from one lot and thereby exempt the other lots as well. This is true even if the field owner owns fields in different parts of town and sells produce from different baskets.
Section two: If one goes to the market and buys from a field owner, if the field owner only brought produce from his own field, then he may separate tithes from one lot for all of the produce that he buys.
Section three: However, if the field owner sells produce from other peoples gardens then the buyer must separate tithes for each lot individually, lest some of the sellers tithed and others did not.