Sorry, but it happens all the time. We get hundreds of posts from people saying here's this knife I want to sell but I'm only asking how much is it worth (can't blame me if somebody makes me an offer). No, you can't do that; too many others have gone before you.
If you really want to know how much it's worth you can search the Exchange forums at Bladeforums, completed auctions at ebay, or if it's really valuable it would make sense to pay our own Bernard Levine for an appraisal. If it's a one of a kind and nobody can tell you, you can list it on ebay with a very high reserve and see what the highest bid is.