One of the best accelerators for the A500, upgradable to 50MHz, shame it only supports (max) 8MB and requires the special GVP simms, however. If you can test it and take photos of it working you'll definitely get more for it than just throwing it up there as "untested" (which usually means "broken"). Document everything, even which version ROM it has in it (you can easily get this info from SysInfo). I've seen regular GVP sidecars fetch $300+ lately. Crazy prices for 20+ year old hardware, but as you probably already know, that's Amiga for ya!
Good luck, and hopefully you'll find a good home for it instead of just sitting on some hoarder's shelf!
