I recently had to assess a collection of photographic equipment held by an art society of which I'm a member and I used a rough-and-ready method: I went to the famous online auction site and checked out the "completed auctions" to find the final selling prices for comparable pieces in comparable condition. It's surprising how often identical or almost identical items show up, even for stuff that's relatively rare. I figured it was the best way to get an approximate idea of current values on the open market.