Maaseros 5:3-4

Maasros 5:3

Once the time to tithe has arrived, a person may not sell produce to a buyer who cannot be trusted to take tithes, nor may one sell produce in the sabbatical year to a person who cannot be trusted to follow the laws of the sabbatical. If the produce ripened, the owner may keep the ripe ones for himself and sell the rest to anyone.

Maasros 5:4

A person may not sell straw or the residue of olives or grapes from the press to a buyer who will extract the liquid from them if he cannot be trusted to take tithes. If the buyer extracts the liquid, it must be tithed but it is exempt from terumah. This is because the one who took terumah from the crop also had these in mind.