Prepare said:
I can list the gamertag "God" on the site and you go to buy it for $2,000, everything checks out, then you receive the info and the tag is "DrunkenCowMeat129".
If...IF the site started here.. because if you tried to advertise it elsewhere people wouldnt do it because lack of trust but
to fix this problem the admin would verify and accept the submission making sure it is legit by speaking to the GT owner, send message via xbl, timestamp, many different ways to go about it i personally would do a "three step verification process"
Basically this site isn't going to be able to run by itself, no auto crap. No way its possible. The BEST you can do is get a group of trusted members here to verify and sell for users and perhaps take a percentage. BASICALLY a MMing service