Demai 2:3-4

Demai 2:3

If someone aspires to become trustworthy regarding ritual purity and impurity, he may not sell either wet or dry produce to an unlearned person (who will render it impure), nor may he buy wet produce from an unlearned person (because it may have already been rendered impure), nor may he be the guest of an unlearned person or host an unlearned person wearing his own clothes (which may have been rendered unclean). Rabbi Yehuda adds that such a person may likewise not breed small cattle (which tend to graze on others’ property), nor tend to make vows or act frivolously, nor may he render himself unnecessarily impure through corpse contamination; rather, he should tend to the scholars in the study hall. The Sages said to Rabbi Yehuda that his additional conditions are not relevant to the matter at hand.

Demai 2:4

The only thing that the Sages required bakers to separate from demai was terumas maaser and challah, which go to a kohein. Vendors are not permitted to sell demai. Wholesalers and produce sellers who sell in bulk are permitted to sell demai.