The easiest thing to do is just refund and move on, perhaps after a final message giving them a day to respond to your liking. You didn't agree to a layaway plan.

I know there are some who will take the "it's principle" stand and say you should keep the deposit but you don't need that shit in your life. You have no idea how much of a potential online psycho this guy could be not worth the potential hassles over $100.

You also might have someone who offered you $100 or so less than this guy when you listed it. Time to send him a follow up message and see if he's still interested.

I will EASILY sacrifice a few bucks if it's going to make my life easier on a sale.