I have many, MANY pets. Quite a few of them are duplicates that just take up space in dorms, attics, banks and backpacks. I hate to release a domesticated animal back in to the wild, and watch it try to make friends with something that will eat it. I also hate the idea of sending it to be euthanized. in the back room of the pet shop.
There are many empty shops in the Shopping District. Why not open one of them up for a S.P.C.A. style adoption center.
I could then take my extra Forest Lord pet and sell it for a few gold (or crowns for the ultra-rare/epic-rare pets), and someone else can come along that really wants that pet, and now they can have it.
Higher prices for trained pets. Regular prices for common/ultra-common.