MS don't allow reselling of anything to do with Live (it's in their TOS), so for obvious reasons it's a no no selling that stuff via the forum.

When you purchase content, you receive a license to use that content. The license has two parts: the system license and the local license. The system license applies to the gamertag that purchases the content, and the local license applies to the console where you first downloaded the content.

A license is another name for media usage rights.

Originally Posted by PenyMan

How does one sell a downloadable game?

Originally Posted by 00Assassin00

Yes, someone before offered me some games, and i saw a thread was selling a lot of steam games, and someone requested a steam game too and they aren't banned or something so, its allowed.
But i think you should keep it in pms or something :/ so, advertise in your signature and ingame because it doesn't have a section here.
Note: you should do business with people your trust only, or ask people about him if you don't know.
If there is any more help you need, just throw me a pm :/