I wrote earlier about Craig's List, the down-and-dirty internet classifieds site that has grown into the dominant player in the online flea-market market. Although it is now a for-profit enterprise, it still retains many of the trappings (DOS-like trappings) of a seat-of-your-pants site.
If you prefer pretty pictures and state-of-the-art web layout ,but still don't want to pay for your classifieds, look to eBay. eBay owns 25% of Craig's List, but that hasn't stopped the auction giant from copying the site. It has created a similar no-charge site called Kijiji.com, which is rapidly growing into a player in this market. All Kijiji content is local, and the company does not get involved in any transactions; no Paypal options, for example. Like a classified, the buyer finds something he/she wants, contacts the owner, and buys directly. I presume eBay will make its money on ads and in segueing buyers to its main site when they don't find what they want on Kijiji.
Although Kijiji doesn't have the folksy cache of Craig's List, it does give a more professional presentation. If you don't see what you want on CL, give Kijiji a look. And remember, caveat emptor!