Related על בבא מציעא 111:10

They did not permit selling Demai except to a wholesale dealer (=סיטון, per Jastrow). A homeowner who sells both this and that (i.e., both large and small quantities, per Lieberman) needs to tithe, the words of Rabbi Meir. But the Sages say, one wholesale dealer and one homeowner are permitted to sell [Demai] and to send it to their friend, and give it to him as a present.

One who purchases from a merchant tithes from each and every [bread] mold (see Menachot 94a:11), and if [the merchant] was bringing [vegetables from various farmers] and gathering them before [the purchaser], he takes tithes from each and every cucumber and from each and every bunch [of vegetables], and from each and every date. One who purchases from a shopkeeper, and goes back and takes from him a second time, even though he recognizes the barrel [of wine], he may not tithe from this [purchase] on behalf of that [prior purchase (see Dem. 5:6)].