You need lots of gold in this scam, probably at least ~50 000 g because of the worth of the mount.
1) Find a legit person selling the TGC mount for WoW gold. This is the most difficult part. I recommend contacting people selling it for cash on Ebay and proposing to pay in gold.
2) Pay and get the code.
3) Resell the code or use it to get a mount with different account.
4) Contact the seller again and explain that the code he gave does not work. Threaten to report him unless he gives the gold back. If he traded the code to you with his main account, he will probably rather give you the gold back than face a ban.
If this doesn't work, you can make a GM ticket and hopefully you get the gold back.