It depends on the value of the coins you have. If you're unloading a few coins that are relatively inexpensive, get a few price quotes from local dealers, or throw them on one of the forums (CoinTalk, NGC, PCGS). (In fact, that's always a good place to start since it's free.) If your coins are of higher value and require wider publicity, consider Heritage, Stacks&Bowers, and Great Collections.