The Marketplace
eBay Motors is a great site to find vehicles of all types from around the nation. It is in an auction format with many listings featuring a "Buy it Now" option that will end the listing with the stated price. Wile eBay may not offer the cheapest deals on average, there are still some excellent deals to be had. Aditionally, eBay offers extensive protection to the buyer ensuring the customer will get the product depicted.
Craigslist offers a real dichotomy. On one hand, Craigslist will usually have the best deals and the cheapest cars; however, Craigslist offers a plethora of scams. You must be wary of fraudulent listing as you peruse Craigslist.
AutoTrader is more of a traditional classified ads website. You must pay to list your vehicle on AutoTrader. This cuts down on spam listings that haunt Craigslist. The trade off is that the prices are typically higher than Craigslist, but the quality of listing is also much higher than Craigslist. There are also a considerable amount of dealers on this site compared to others.
CarGurus is very similar to AutoTrader, but it offers one important distinction to set it apart. CarGurus offers pricing research as well. They will compare the price of the car you are looking at to the price of similar listings.
TrueCar is very similar to CarGurus. It offers similar pricing research to CarGurus. The only reason to browse both would be to see a variety of listings. Thy could offer different listings, so you might as well take a look at both
Bring A Trailer is a much more specialized website. The person who is listing their vehicle must go through a through vetting process to ensure that the listing is accurate and that the vehicle has some sort of significance. As a buyer, you should check out this site if you have specific needs or wants. Many of the vehicles listed here are collector cars.
Facebook Marketplace is not just a social media site. They offer similar services to Craigslist without all, or most of, the spam. The listings are almost exclusively by private sellers and not dealers.