I always list machines I'm selling in the Pinside Marketplace, along with Craigslist, and sometimes the FaceBook group Pinball Machines for Sale.
Craigslist got me a sale on Starship Troopers and Spring Break, both excellent buyers, no issues. But lots of tire kickers and low ballers came from the Craigslist crowd. Pinside helped me sell Hot Shots last week which was also listed on Craigslist. Turning the Marketplace ad into a forum topic helps get more exposure, but be ready for some occasional ignorant comments, the price police, and any pictures of the game being gone over with a microscope, depending how you list and price it. FaceBook has nice comments and feedback, but hasn't done anything for me other than that.