First determine the value range of the item you wish to sell. Do some on line research, browse some of the local gun stores selling used guns and check out some of the used gun value books. That will give you some idea about what you can ask for it.
Gun stores will usually give you 50 to 60% of what you can sell it for in a private sale. This isn't ripping you off, its what they are willing to pay in order to pay overhead and turn a profit. Some will try and low balling you to perhaps 25% or less of what they'll sell it for later, esp. true of Pawn Shops, but you can always walk away from those deals IF you know the approximate current used value of the pistol.
If you decide to sell it privately then its going to take some work and maybe a little investment in advertisement.
You can advertise on this forum for a FTF (face to face) sale. You can put a 3x5 card on the bulletin board at most gun ranges with firearms for sale. Gun shows have been mentioned. Tell friends and associates that your want to sell the pistol, word of mouth works quite often and its cheap.
Not sure if the news paper will take pistol adds in Washington being its such a liberal nanny state, esp in Seattle but there's likely other advertising sources like penny saver or other media.
You can get an account on Gunbroker and put it up for auction with a reserve price.