The prices in the online classifieds here are all over the map. Sometimes the cars don't even exist, the car is stolen, or the seller doesn't have the right to sell the car. I once found an excellent deal--a steal, really--on a Mercedes on SuperCarros or a site like it, and ended up swindled out of a deposit (with the help of a shady gringo realtor in Las Terrenas).
Every city has local people, mechanics usually, who sell cars part-time. These guys know what cars are really worth in your area. The trick is to find one that you can trust, which means referrals from at least two people who have used their services in the past. In the end, the value of my car ended up being close to what I thought it was, which was the KBB resale price (not the much lower dealer wholesale) in the USA plus a small island premium. I started by listing the car here and elsewhere, as well as putting a big sign on the back window. I had lots of local people look at it. I got outrageously low offers or promises to buy, pending financing. I confessed to a gringo friend who had lived here for ten years that I was getting frustrated enough to consider shipping the car back to the USA (for $750) and selling it in Florida. He said, don't do that, I know a guy who always has buyers and will sell it for you in a day. The seller, who owned a repair shop, negotiated me down a little to make room for his commission (around ten percent, I'm guessing), put a new battery in the car, and I'm supposed to be signing papers and getting cash later today. PM me if you need more info.