I think AK has it perfectly.
As to why this is on the milsurp forum and not general discussion...well it's milsurps that this becomes most important. Modern items can be easily valued - but try pricing out something old or rare. I think I have an above average knowledge of prices but even I have to scratch my head as to what's a fair price to pay, or a realistic price to sell, and I use the EE to search for past sales. But it seems many are in a rush to keep it hush. We can work hard or we can work smart.