I use Craigslist. Have both bought and sold bikes. Craigslist is free and you can alter or delete your ad any time you want. You can take a few pics, write your ad and have it on Craigslist in maybe 15 minutes.
Yes, you may have to deal with a couple of idiots, no big deal. And besides having interested persons contact you through Craigslist email I also list a cell number, lots of folks do a lot of texting these days. Besides a few good pics, the more info you can put in your ad the better, will save you time answering the obvious questions. And this forum has had discussions regarding how folks feel about allowing test rides--I'm in the category that it is highly unlikely I would allow a test ride but if a prospective buyer wants to make sure it runs, shifts, brakes OK they can ride as a passenger for a couple of miles.
Craigslist has a policy of one ad in one location. But when I am really bored sometimes I look at their bike ads and besides here in Louisville I may also look at their ads in other cities. I see a LOT of bikes advertised in more than one location. It appears that if you alter the title of your ad even a little bit they won't pick up the same bike listed in a different location or two.