What can be sold here:
Xbox Live accounts
Gamertags (Gamertags containing "vulgar" words are forbidden)
Xbox Live cards (Codes received from Customer Support are forbidden)
Microsoft Points cards (Codes received from Customer Support are forbidden)
Account services (Modded lobbies must be verified, see sticky HERE)
I don't think customer support codes aren't allowed to be sold here. Sorry.
http://www.sythe.org/showthread.php?t=1207494
1 Reason they may be banned is because they may have been received falsely, such as social engineered, which is illegal. If the codes were to get reversed by microsoft because they found out that's what you did, then the person who bought them's account could be in jeopardy. That's why.

