You have a complete M49 Spotting Scope kit? The whole kit is worth several hundred dollars easily to someone looking for one.
My gut says that if everything is functional and in good condition 500 dollars is the bottom end of what I'd consider paying for the set. There has been a lot of interest in "Historic Sniper" setups lately, including several well known gunsmiths marketing M40 replicas complete with refurbed ART scopes.
Machine guns are awesome until you have to carry one.