Sheviis, Chapter 5, Mishnah 7. Our mishnah deals with the potter and the question of to whom, and in what quantity, he may sell the earthenware vessels he has made.
The text of the mishnah:
"HaYotzer mocher chamesh kadei shemen va'chamesh esrei kadei yayin, shekein darko lehavi min hahefker" - the potter may sell five vessels designated for oil and fifteen vessels designated for wine, and he may do so even to someone who is suspected regarding sheviis, that is, suspected of not keeping the laws of the Sabbatical year. The reason: it is normal for people to gather such a quantity of oil or wine from ownerless produce, which belongs to no one, and therefore we are not required to assume that the buyer is doing anything improper.
And how do we know that the buyer will not mix the vessels, using the oil jugs for wine in order to bring in more than the usual amount from ownerless produce?
"Kadei shemen" are vessels built in a manner suited to oil alone; people do not normally use them for wine, and likewise the reverse. For this reason we are confident that he will use only fifteen for wine and five for oil.
It is worth emphasizing: if a person happened to succeed in gathering more than this quantity from ownerless produce, that is permitted. It is simply that this is not the normal practice, and since it is not the norm, the potter may not sell more than the stated quantity.
Two exceptions, in which it is permitted to sell even more than this quantity:
"U'mocher l'goy ba'aretz" - it is permitted to sell to a non-Jew even in Eretz Yisrael, since he is not bound by the laws of shemitah.
"U'l'Yisrael b'chutzah la'aretz" - it is permitted to sell to a Jew outside of Eretz Yisrael more than this quantity, and we need not be concerned that he will bring the vessels back to Eretz Yisrael, since his work is done outside the Land.
In summary: in this mishnah we learned that the potter may sell even to one suspected regarding sheviis five jugs of oil and fifteen jugs of wine, because such a quantity is normally gathered from ownerless produce; and since oil vessels and wine vessels are not interchangeable, there is no concern that he will bring in more than that. We further learned that a larger quantity may be sold to a non-Jew even in Eretz Yisrael, and likewise to a Jew outside the Land, where there is no concern that he will bring the vessels back to the Land.