People are lowballing you 10 keys. Obviously it's worth more than that if they are offering only 10 keys for it, otherwise they would just pay someone's buyout and save the hassle of making the same offer on 100 trades and being turned down everywhere. They are lowballing so they can resell it for higher obviously. I paid around 16-20 keys and I'm happy to say it's worth 14-18 keys if it means getting a price on this thing. I'm pretty sick of people undervaluing this weapon though, it's absurd. It's probably the best weapon in this game if you actually have some skill and can aim for the head.

Upvoted since I just got one for about 11 keys in items and saw other similar offers around.