I also suggest you post on all of his threads selling this Geo+ that the gun is sold to you, just so that he has no option of selling/trading it to someone else while you're trying to resolve this.
Contacting his teammates on Texas Grind is another way to put pressure on him to respond. I think one of his teammate is Darius09
(David). Politely ask if he knows of the seller, and if he would contact him.