Craigslist has a section near the middle of its homepage called "for sale" and a link option beneath it called "atv/utv/sno" (short for "atvs, utvs, snowmobiles"). Selecting this link opens a list of links where Craigslist users post descriptions and photographs of 4-wheelers and other off-road vehicles.
Reading through several descriptions can give you an idea of the model types and prices common to your local area, as well as the prices for which 4-wheelers of different brands and model years are for sale.
If you are ready to make a purchase, or just ask a 4-wheeler owner a question, Craigslist allows you to contact the owner by way of a grey button marked "reply" on the top left-hand corner of their post. You can write to the owner about small details that they have not included in their post or inquire about their availability to meet you to try out and possibly purchase their vehicle.
If you find that your particular area does not have a selection of 4-wheelers that entices you, return to Craigslist's home page where, near the top right corner, you can see a link called "nearby cl" (short for "nearby Craigslist") which offers links to the Craigslist forums of areas in the general vicinity of yours which have their own section containing 4-wheelers for sale.