Sites such as Car Gurus, Autotrader and CarsForSale.com contain listings for used Mazda RX-8s sold by dealers, which are inspected for any issues before being sold. CarMax also sells used Mazda RX-8s and offers a guarantee of quality.
Used car dealers are legally obligated to inspect all cars before being sold, as well as inform buyers of any major issues with the car before hand. Specifically, CarMax performs an extensive inspection on all of its cars to guarantee the best performance. Private sellers are not obligated to do so and are not required to do anything if the buyer experiences an issue.