Most states limit the number of cars that an individual can sell without a dealer's license, or only allow the selling of one's personal car. "Curbstoners" (or "curbsiders" in Canada) are people who ignore these laws and sell multiple cars that frequently have hidden problems in their pasts -- problems that can affect both the safety and the value of these vehicles.
Before buying any used car, you should research the vehicle and the seller. Try to get a vehicle history report.
Be wary of sales conducted from the side of the curb or a vacant lot. Often these vehicles are sold by con men posing as private individual sellers.
When buying a car from the classifieds like Craigslist, do a web search for the phone number and see if any other cars are being sold by someone at the same number. If there are, it's likely a curbstoner.
Title jumping is illegal. Curbstoner sellers due it a lot when they are scamming people into buying bad car and cars that can't be titled to the new buyer.
Oftentimes it is people who are car dealers but do not want to go through the legal issues to become one. I was a curbstoner for years. I was a licensed dealer but I sold cars "on the curb". Not to cheat anyone but to have lower overhead & operating costs.
Under the Massachusetts law, a dealer is anyone who sells four or more vehicles in a 12 month period.... So if your NOT a Dealer and you sell 4 or more cars it's considered a MISDEMEANOR... SORRY....