I've never seen a business that would but used parts... at least in this neck of the woods. I have used some used parts as a bargaining chips before. There was a used car dealer that had a couple of Bonnevilles in a fenced-in part of there lot. Also on there on the lot was a 95 SSEi with a shattered RF fender. I traded a fender for a RR door for my car from one of the parts cars he had. Both of us were happy with the transaction.
I would say that Ebay would be the best place to sell those too. It would be nice to know exactly what other card those seats fit so you can sell to a bigger audience.