"He said, great, it's yours..."
If that is what he said, then honor demands that he keep his word. It is a question of a man (or woman) being honest.
When I said that my policy is basically, "No money, no deal," I meant that I do not say, "Great, it's yours" until the money (at least a deposit) is in my hand. If that makes me mercenary, well I guess I may be. However, I've just had too many people say, "Hold it for me and I'll get the PayPal to you tomorrow!" only to never hear from them again. So, I'll hold an animal for you, but only after some consideration (money) has made it my way.
Given the givens here, though, Brandon I'd have to say that you were wronged by this individual's lack of ethics. I will sell a snake if a higher offer comes in before I get a deposit from someone, but I will go back to the first "bidder" to see if he wants to match that higher offer. But, I never say "it's yours" until it's sold, and it isn't sold until I get some money. At least, that's my general practice, I might ease up with someone I know really well, but not a stranger.