Well, you got actually no proof if he's the real person who sells that acc/ign...

For example if I hate someone I van easily type "Sell account Name 'xxx', officer of guild: 'xxx'" altho I'm not even that person.. only ingame ss's can lead to real proofs, those 3rd Party chats aren't trustable.

Selling/buying of gold/accounts is on own risk, many get scammed by those ways so actually it's their own stupidity tho.