Sheviis, Chapter 6, Mishnah 4. The Mishnah addresses the question: from what point may a person buy vegetables once the Sheviis year has ended - that is, from when may we assume that they are not produce of Shemittah, but rather from the new crop that grew afterward?
The Mishnah's answer:
From the time that something similar to the item being purchased has formed or grown. The buyer is not required to know with certainty that this particular item grew after Shemittah; once a similar species has grown in general, it is permitted to buy, since we may assume that it grew after Shemittah.
Bachir and afil:
The Mishnah continues along these lines: in a region where the crop tends to ripen quickly, once it has grown there, even "ha'afil" - the crop that grows more slowly - becomes permitted, even though in the region of slower growth most of the crop has not yet grown. Its very presence in the other region is enough to assume that it is permitted here as well.
"Rabbi hitir likach yarak b'motza'ei sheviis miyad":
Rabbi Yehudah HaNasi permitted buying vegetables immediately after Sheviis, and two reasons were given for this:
A considerable quantity of vegetables was brought into Eretz Yisrael from outside the land, and therefore one could assume that most of the vegetables available for purchase in Eretz Yisrael had in fact come from outside the land.
Certain vegetables cannot grow within two or three days; therefore "miyad" as stated here does not mean literally right away, but rather after a few days have passed - it is enough that there be sufficient time for such a thing to grow.
In summary: in this Mishnah we learned that permission to buy vegetables at the conclusion of Sheviis depends on a similar species having already grown, and no detailed knowledge about the item itself is needed; that the growth of the early crop in one region permits even the late crop in a region where growth is slower; and that Rabbi permitted buying vegetables "immediately" - because of the abundance of vegetables brought in from outside the land, and in a manner that allows enough time for such a thing to grow.