Maasrot, Chapter Five, Mishnah Three
The first three mishnayot of chapter one taught that fruit is not liable for tithes until it has reached a certain stage of ripening. Our mishnah teaches that after it has reached that stage one may no longer sell it to a person who is suspected of not tithing.
1) One may not sell produce after the season for tithing has arrived to one who is not trusted concerning tithes.
2) Nor in the sabbatical year [may one sell sabbatical year produce] to one suspected of [transgressing] the sabbatical year.
3) If only [some] produce ripened, he takes the ripe ones and may sell the remainder.
Section one: Once produce has ripened enough such that it is liable for tithes, one shouldnt sell it to an am haaretz who is not trusted to tithe the food before he eats it, or gives it to others.
Section two: Similarly, while it is permissible to sell produce that grows during the sabbatical year, one shouldnt sell it to a person who is not trusted to observe the laws of the sabbatical year that apply to that fruit. The most important of these laws is that once that particular fruit can no longer be found in the field, the fruit must also be removed from ones storehouses. The seller must be concerned lest the person to whom he sells the produce will hold it in his home beyond this date.
Section three: If some of the fruit has ripened and become liable for tithes, but some of it is still unripe, he can take for himself the ripe fruit and he can sell the unripe fruit to anyone, even one who is not trusted to tithe. The ripe fruit can only be sold to someone who is trusted to tithe.