The easiest place to find a used four-wheeler or ATV for sale is at a local ATV dealer. You can also look for them on online sales sites or classified listings.
ATV dealers typically sell both new and used four-wheelers. Even if the first dealer you contact does not have used vehicles for sale, he can tell you who does. These ATVs might be more expensive but they also typically come with a warranty and are free from mechanical problems.
If you belong to an ATV or motorsports club or know someone who does, checking with them may be a good approach as well. It may help you locate ATVs that have been well-maintained and may not have been publicly listed for sale yet. These sellers often have extra incentive to be honest because most clubs are fairly small and they do not want to get a bad a reputation by misrepresenting their vehicles.
Classified ads and online auction sites are also good ways to find private sellers. Be sure to do a full inspection for signs of rust or damage if you go this route, because there is no way to know how well the ATV has been maintained. Craigslist is often a good way to find local ATVs, and if you are willing to travel, there are some specialty sites, such as ATVTrader.com.