There is nothing inherently unsafe about installing salvaged auto parts in your vehicle, including seats, according to eBay. Make sure that the seat isn't bent out of shape and that it fits your car.
Ebay mentions some additional things to consider. A salvage seat needs to be one manufactured for your specific vehicle, with all of the same mounting points, electrical connections and adjustments, whether powered or manual. Also, if the salvage seat includes a built-in side airbag, it should be inspected and replaced if necessary. If aesthetics are important to you, find seats that match the existing colors and material of the car's interior.