Pay me $300, and I will take your unwanted animal. No need to feel guilty giving it to the pound or having it put down - give it to me, and I'll keep your animal. (And, well, if I decide to put it down in the end, it's not on your conscience!). I could make a pet store, or have any number of things.
I would guess 1 in 8 customers will have a mother or father come in and give me their pets in the morning, only to have them come back in the afternoon with a crying child asking for it back. Well guess what? This pet has become very special to me, and I wouldn't think of selling it for less than another $300.
I think people everywhere could benefit from my business, and I will be a raging success. Running the first round of investors now.