Here's my advise, if you really need to sell something urgent and for your case there's an interested buyer who have already paid a deposit but somehow is delaying the full payment and collection, best is if you refund back his deposit and find another buyer.

Of course do let him know in advance that you need to sell it urgent and therefore you are refunding his deposit to find another buyer.

Or another solution is to ask him to make the payment in full then you don't have to worry so much, then it is up to him whenever he want to collect the item/product.