If a kosher slaughterhouse is in the habit of selling the hindquarters, isn’t that in violation of doing business with non-kosher foods ?
Is it perhaps because the only part of the hindquarters that is actually forbidden to do business with is the gid hanasheh and that has no value ?
One of the heterim to sell non – Kosher items is that the prohibition is only if the person prepares himself to sell the item, but not if a non- kosher item happens to come a person’s way. This heter is called nisdamen. This is the reason why slaughterhouses can sell animals that are found to be treif- because they didn’t originally plan to sell the trief animals, rather it happened, that they have it. Therefore, they can sell them. Some hechsherim rely on this even for gid hanasheh and selling the hind quarter, the rationale being that they aren’t in business to sell hind quarters, rather this is what they can’t use, so they are selling it to gentiles.
To add to this, the opinion of Tosefos (Chullin 89b D:H V’hilchosa) is that since the gid hanasheh has no taste, therefore the gentile isn’t paying for it, and the price he is paying is for the rest of the hind quarter. Also see Minchas Chinuch 3-14. Regarding the fats next to the gid which we don’t eat, they are only assur m’drabonon, and the prohibition not to do business only applies to things that are assur m’dorayso.
As a side point, the Eida Hachareidis, does not sell the hind quarters of the cows they shect, rather the cows are owned by gentiles, and whatever they want to buy they buy, and the hind quarters are simple not bought by them.