Huh, never even thought of that. I will actually do that, thanks for the great advice!
My friend sold a DLC skin for a fighting game (injustice).
The buyer said it itbwas redeemed. My friend took a video of his account. He showed the email thats tied to his gamertag.
Then he booted up the game, and took a video of the character selection screen. The dlc skin is not there. Plus the dlc is not in my friends download history. So he didnt redeem it.
Your buyer might make up an account. So tell him to go to the account settings, and have him show you the email thats tied to his username/gamertag.
And if its a new account, it wint have achievements, so its fake.