Used truck tires are generally sold back to the tire shop if they still have some life to them. The tire shop then fits new treads on them, and re-sells them to the public. The perform just as well as brand new tires for a time, but cost up to half the price.
Used tractor tires on the other hand are tires taken off of tractor when the owner bought new ones. These tires are sold as-is. But you don't have to worry about the quality and safety of the tire because the tire shop will guarantee their safety.
