don't know that I can offer any real help, but.......
I bought some nice vintage gtv sparco seats on ebay some time ago. At the time I needed something like what was offered. The minimum bid was about $450 (rears included). I was the only one who bid. I prolly spent to much.
If you find out how much they would be to have recovered, and subtract that from $500, that should give you a starting point.
That is if the mechcanical parts of the seats are in good order.
If you put them up for auction as they are, you might get $99.
That is a guess. Figure it is worth what it cost you.