Estimating Market Value, rule of thumb?

Hi!
Is there a general rule of thumb regarding asking prices of used furniture? I mean we'd all like to think that our items are mint---but still, it is just used furniture in the long run---unless someone is truly looking for a particular piece for their collection.

Someone on the site FurnitureWorldMagazine a few months ago told me that the stuff depreciates rapidly and ten years or so out may only be valued at about 10-15% of the original cost. From what I have seen for items that have sold, this seems to be true.

Does anyone have any ideas? I am trying to come up with fair market value on my entire Radius collection that was purchased in 1996. Thanks!