I see no reason to refund the deposit. A deposit is good faith money to hold an item rather than sell it to someone else. If the buyer is the one who cancels the deal (by word or action), the deposit is forfeit. I don't know of a place that doesn't work that way. I know there's been times that I've taken a deposit on a laser job and the guy has gone and sold his gun. I certainly don't give the deposit back in that case. For me, it's payment for my time in design. For you, it's payment for holding the gun through a time that you may have missed other potential buyers while it was off the market.
Email him advising that he has one week to complete the deal or his deposit will be forfeit and the gun will be relisted. I understand it's awkward if there wasn't a specific time frame agreed to for completing the deal, but 2 months is more than enough.
IIRC I had a similar situation where I held a gun for a month or so for TX_Aggie on his word of good faith and he disappeared. It seems to be a pattern.