Of course! Once you've registered it, it's yours.  You can sell it to whomever you like. In fact, if you have a good domain name, reselling it could be quite lucrative.  The domain name business.com recently sold for over 7 million dollars!  Buying a domain name can be more tricky though as it may have certain caveats.  It can now be argued that a common brand name is entitled to own its corresponding domain (e.g. McDonalds.com)