I must say that there have not really been alot of Axe sold on the secondhand market. I recently sold my spare for £300 and have seen a few on here between £250 to £300. The boss one at £500 is the most expensive secondhand one I have seen.
I think that the reason there have not been many is because they are a really good and cheap marker. Our team uses them and we have 16, with the exception of the odd barrel break we have had zero problems in over a year of use, that is the real reason that I sold my spare.
Good luck looking for one, and can assure you that they are a really good marker and worth the investment - for their small price they keep up with the high end ego/geo/creed type of markers.