If you're in a city with an active Craigslist bicycle section, and if you're comfortable dealing with strangers, Craigslist.

I've sold 2 bikes on Seattle CL in the past year, each in a day. I sold a rooftop bike carrier the other day in 30 minutes. All were priced right, well described, with good photos. By "priced right," I mean - fair price to the buyer, fair price to me.

There are fakes and goofballs trolling Craigslist, but plenty of real buyers and sellers as well.

If you've got a valuable or unusual bike that's worth exposing to a national audience, try eBay.