I would take it to your local stang store and trade it for something. By the time someone paid the shipping it would put it over it's value. Unless you find a local who would give you 200.00 if its in good shape, front and back.
While they don't have a ton of value, someone out there is building a car from a basket case and needs one.
The coupe back seats are one thing that never changed from '65 through at least '70, so you have a big audience to choose from. The only difference in them was in the upholstery applied to them. The seat frames and buns never changed.
I would put it on the free board.Personally I think it's one of those parts that if someone needs it maybe you'd get about $50 for it,but you may have to hang onto to it for another 10 years.I don't see them bring good money on Ebay because they are plentiful and the shipping would be killer.2+2 and Convertible back seats are worth hanging onto though.And back seats do take up some space.I know how it gets I've been doing some parts house cleaning myself.