Avodah Zarah 15 - Animal Laws
In a place where the people follow the custom to sell small livestock to idol worshippers, one is allowed to do so, and in a place where the custom is not to sell small livestock, one is not allowed to do so. The reason for this rule is that in the first place, idolaters are careful to guard against bestiality, but in the second they are accustomed to permit it.
But contrast this with the following ruling: "One should not stable an animal at the inns of idol worshippers, because they are suspected of bestiality" - this ruling gives a blanket prohibition! The difference is that the idolater is concerned that his animal would not become sterile, and will not sodomize it if it is his own.
One is never allowed to sell large livestock. The sale may happen right before Sabbath, and the idolater will ask the Jew to call the animal to see how it walks, the animal will move and carry its load. The Jew will thus cause his animal to work on Sabbath. Therefore, to sell through an intermediary is allowed - since the animal does not know the reseller and will not follow his command.
Art: Govert Dircksz. Camphuyse - Herdsman and Herdswoman with Livestock