The sad fact of the matter is that plenty of suckers are willing to overpay for used markers. For that matter plenty of suckers are willing to overpay for new markers.
MCBs BST isn't as bad as say... CL. Still I see enmeys and hammer 7s priced within a fin of retail, after shipping you may as well walk into the retail shop and purchase the fresh. Perhaps these blokes just like to haggle.
Paintball participation isn't growing as quickly as the stock of new and used markers. Give it some time, eventually the "collectors" will begin to hemorrhage their not-so-rare pieces.