This is a 14.3", unprepared, fossil sauropod dorsal vertebra from the Morrison Formation in Colorado. It is the body (centrum) and vertebral arch. The processes are missing. It is being sold as is and would make a nice project for someone who wanted to try their hand at specimen preparation or just likes the look of unprepared bones.
There are a number of sauropods known from the Morrison Formation. The most common were Camarasaurs, Diplodocids and Apatosaurs, but there are at least a dozen other genera.