Any that have duplicates (full grown/ rarity is listed) are free to take for the average sell price. If I only have one of a specific pet, I guess just make an offer. I made a folder for some of the onlder ones, it's not finished though.
I really like these new alien cats, so I'm saving up for them. Idk the prices of the pets I have, so I mean you could lowball, I wouldn't mind that much.
If you want to offer another pet instead of C$, I might decline your offer. But, again, still figuring things out a little.