I know it could be challenging,
but something that might be fun to have is a "lost" icon.
This would appear on released pets in the wild, so you could know if there was a previous owner!!!

Also when using advanced search on pets,
a "previous owner" option might be fun,
so maybe we could see how many pets we have sold!!
-and for the people that get upset when there pets sold higher than what they were bought for, they could make sure no-one is doing so-

As far as I know released pets don't actually go back into the wild, so that wouldn't be applicable.

I have considered a previous owner button, and I do think it would be nice to have, but for pets that have gone through lots of homes it seems like it would be a lot of trouble, and I'm not sure how feasible it would be.

I understand that some people get upset when people do that, but there's nothing actually wrong/against the rules about it and people shouldn't be harassed for it, and if that feature would be used for it, then it completely loses my support. Once you sell a pet it is no longer yours, end of story. The new owner can do anything they want with it, including selling it for a higher price.