Better watch out. They can make you sell wether you want to or not and whether it is copyrighted or not. Just ask the guy who used to own wallstreet.com. He was forced to sell even though he did not want to. Wallstreet is not trademarked or copyrighted.
Yes, it is perfectly legal. Thousands sell everyday. You do not need a lawyer, although if the dollar amount is high enough, you may wish to contract with one. The problem is that, since it is such a new area, most attorneys are clueless.
In any event, use an escrow service to handle the transaction. It is worth every penny of the fee (usually between $250 and $500) to make certain that you don't get screwed. This also protects your buyer.
Register.com has an escrow service, and they are VERY experienced in the domain business.