As a seller I wouldn't do a 50-50 in this market unless you are getting >FMV sales price.
Here's an out of the box idea. I imagine this is all an issue of trust as is usually the case. Y'all two find some common ground. Maybe instead of doing 50-50, escrow, or having 3rd party "repair" the gun think about meeting in person. Split the airfare. Say $250 ea. He's flies over one morning, y'all go shoot, hang out, eat lunch, get all comfy (or not) with each other and then he flies home. Done. 1 day. $250 ea. and he gets to finger the gun and y'all get to know each other. Sure, sounds a bit weird but it's really not. On the surface it sounds quite reasonable to me. You pay $250 to close the deal in your terms easily. He gets to handle the gun in person and make sure you aren't a scammer. $250. A pittance.