Transactions

You really can't. If the seller paid for a "certified pre owned" (CPO) test for their vehicle (usually through a dealership) then you have some assurances it's as advertised, but those CPOs are only as good as the organizations signing them. If no CPO and it's an older car, then you need to rely on your abilities to evaluate a car, or have a mechanic give it a once over. Always ask for maintenance records. Familiarize yourself with the vehicle's service intervals (mileage at which certain major servicing should occur, like transmission servicing every 30K, or a water pump or timing chain at 100K, etc.), then check of those have been performed.