Your thread is not in the proper format for the BOI...but it appears that your question is better suited to the General Business Discussion; so it has been moved.
Personally, I would refund their money & cut the tie...but that's me. When I was selling, my terms included a statement that defaulting on a payment plan would result in a forfeiture of monies paid...but I never stuck to that. IMO, it isn't worth it. Better PR to give the refund, even if you don't plan on doing business with them in the future (and if somebody defaulted on a payment plan, I wasn't going to be selling to them again - just the way I am).
If you want to let them try to sell the animals to pay you - that's fine; but I wouldn't count on that happening. (Think about it - why should they bother to sell your animals for you?) Let them try, if they're willing; but you should proceed with advertising them.
As far as refunding, while I wouldn't bother with it, you could withhold a portion of the funds for the care and maintenance of the animals...but there are issues to consider. First and foremost - you would have been caring for, and maintaining those animals anyway; so why try to charge for it.
Can you say, with confidence, that the animals would have already sold had you not entered into this deal with your buyer? If not, don't use that as an excuse to try to keep their money.
Consider the given reason that they failed to complete the purchase - they don't have the money. Do you want to be the person that takes away from somebody that could truly be in need? (Yeah - the explanation could just be words on a screen; but what if it isn't?)
If they paid via PayPal, in all likelihood, they can get their money back by filing a claim. You didn't ship, there's no reason for you to have their money. You can try to argue it, based on payment plans, incomplete payment, etc...but I doubt it would change the end result. All it would accomplish is a delay and some aggravation on both sides.