When buying a used car first thing you need to check is the "car-fax" auto history. Another similar option would be to get a Vehicle History Report at Autocheck.com. This sort of reports will tell you everything from who has owned the car, how many times the VIN number has passed through any DMV, to what kind (if any) damage has occurred. Examples are accidents, water damage, theft, and odometer readings.
You should ALWAYS have the car checked out by a professional mechanic to make sure there are no obvious defects.
Also, there are cheaper, safer options than buying used cars either from private owners or directly from car dealers: buying used repossessed cars for sale at public auctions. Repo cars are vehicles whose original buyers defaulted on their auto loan payments, and therefore banks regained possession of them. At these repossessed car auctions, you can sometimes purchase vehicles in very good conditions for 50%-90% less than their street price. In fact, many car dealers buy used cars at repossessed car auctions and then resell them for a profit at their dealerships. Repossessed Cars for Sale -