Just go to the gamertags section and post what you want to buy/sell in there. If you're selling a tag make sure to * out some letters because it could get banned. If you need help figuring out what something is worth there is also a section for appraisals. That's pretty much all you need to know.
Home -> Marketplace -> Traders Market or Gamertags -> New Thread post what you want to trade or sell. Go to buyers market to request a gamertag or anything you want.