Huh. I'm in the exact same situation with this guy. He's paid $200 and owes $280 more on a gun. I tried to PM him here a week or two ago and was met with the same "inbox full" problem. I sent him a PM on Nation and haven't heard anything.
I was leaning toward refunding him and just listing again, and I'm honestly a bit surprised to see people here of the opinion that his payments are forfeit. For reference I've been hanging on to the gun for like... 5 months now. I had told him there was no rush, and there really wasn't, but I'm growing impatient at this point.
The other problem is I've successfully sold this guy stuff in the past. Weird.