This mishnah deals with the practices surrounding the sale of animals to non-Jews. The basic principle that emerges is that one may not sell a large domesticated animal to a non-Jew - a cow, a horse and the like - and at the root of the prohibition stand two concerns that the Sages had.
The two concerns underlying the prohibition:
The concern of lending and renting: Perhaps instead of selling the animal to the non-Jew, he will lend it or rent it to him, and the non-Jew will use it for work that is forbidden on Shabbos. The non-Jew himself is permitted to do work, but an animal that is owned by a Jew may not perform work that is forbidden on Shabbos, and this is because of the Jew's ownership. It turns out that if he only rented or lent the animal, his animal is doing work on Shabbos.
The concern of a sale on Erev Shabbos: Perhaps he will sell it to the non-Jew close to the onset of Shabbos, and the non-Jew will want to test the animal. Sometimes it is difficult to get the animal to move: they place a load on it in order to test it, and sometimes the owner shouts at it, and it recognizes his voice and moves because of that. The concern is that right before sunset the non-Jew will place the pack saddle on the donkey, and the Jew will say "Giddy up," and the donkey will walk - and this is driving one's animal on Shabbos, since the animal is carrying a load because of the Jew's actions, before he has actually sold it to the non-Jew.
The question of the mishnah - small animals:
The question the mishnah addresses is a question of custom: did people adopt the practice of not selling even small domesticated animals, such as sheep, which do not perform work? In other words, are they included in the decree that was enacted because of the concern regarding large animals, or not? The mishnah rules that the matter depends on local custom:
"Mochrin" - in a place where the custom is to sell non-Jews small animals that do not perform work, it is permitted to sell them, since this is the custom.
"Ein mochrin" - in a place where the custom is not to sell small animals that do not perform work, one may not sell them, since this is the custom.
A large animal - everywhere:
Regarding a large animal, however, the mishnah rules that everywhere "ein mochrin lahem behemah gasah" - one may not sell them a large animal, for the reasons mentioned. And included in this are "agalim u'seyachim" - even young large animals, such as calves and colts (the term for young donkeys). Even though they are not yet capable of performing work, since they are of the same species as large animals, they too were included in the prohibition.
Also included are "shleimin u'shvurin" - whether they are whole and unblemished and capable of work, or whether their leg is broken and they are incapable of work. The reason given: there is a concern that one might confuse it with an animal whose leg is whole - and just as an animal that can perform work may not be sold, so too this one, whose leg is broken and which cannot perform work, may not be sold.
The view of Rabbi Yehudah:
Rabbi Yehudah disagrees and permits selling an animal whose leg is broken, for two reasons:
It is clear that no work will be done with it.
Even the concern of confusing it with an animal whose leg is whole does not apply, since a person hurries to slaughter it and does not keep it in his possession, and therefore it is not seen for any length of time.
The view of Ben Beseira regarding a horse:
Finally the mishnah brings that Ben Beseira permitted selling a horse. His reason: a horse is not a beast of burden, but rather an animal that carries people, and a person carries himself. We have a principle that "a living being carries itself," because it shifts its own center of gravity, and therefore one who carries a living person - and even an animal that carries a living person - is exempt for the carrying, and this principle applies here as well.
Since a horse is used for a person to ride upon it, one who rides a horse on Shabbos does not transgress, because of "a living being carries itself." And since even if he has a person ride on it on Shabbos there would be no Torah-level work involved, Ben Beseira permitted its sale. And one must be precise in his wording: his permission is for the sale of the horse alone and not for riding it, for any concern of a person riding on it is not a Torah-level act of work and is not a Torah transgression.
In summary: In this mishnah we learned the prohibition of selling a large animal to a non-Jew and the two concerns underlying it - the concern of lending and renting, and the concern of driving one's animal on Erev Shabbos. With a small animal the matter depends on local custom, whereas with a large animal the prohibition applies everywhere, including calves and colts, whole and broken. We examined the view of Rabbi Yehudah, who permits an animal with a broken leg, and the view of Ben Beseira, who permits the sale of a horse on the strength of the principle "a living being carries itself."