My guess, from the stamp, is that it's a Mexican bowie! (I know, right?) Age is impossible to determine from the pictures. I'd be leery of paying too much, considering how easy it is to bury a newer piece in the ground for several months, clean it up some, and claim it to be older than it really is. If there is some provenance to go with it, however, you'd be in a much better position to know if your price is right. Any details you could provide might help with an accurate dating of the piece.