Have you checked eBay's completed listings? That always helps me to determine a market value.

I've bought items from members of this forum and then noticed the same item on eBay with a "Buy it Now" option for a substantially higher price. I usually contact those people and say "FYI, I bought this for from someone on a forum, just so you have a reasonable idea of the market value". Most of the time, they are grateful for the info.