The worst thing is that doing this can be very dangerous. If you use a registered trademark or copyrighted name, then try to sell it to the holders, they can sue you for infringement on the domain name. Remember the story about the family that bought harrypotter.mob? they got forced to give it up when they were trying to sell it for £2,000!
Still a lot of money to be made in domain names for those that knows how to play the game. See what fly.com sold for recently on SEDO. I wish i had that domain.