The best way to do it, go to Ebay, start the auction from your minimum, like $300 or any number that you are comfortable and then see how high it would go, that's it.
Post your ebay links on all 3 Amiga forums and watch it go.
Test it first. Maybe it's been upgraded internally with Rejuvenator II, with better CPU and RAM, you'll never know unless you test it.
Or maybe it's broken and someone just put it in the box and store it in the attic.