1. Old kit is recirculated, meaning sales are not made by the industry to those players - sales lost now, and also in the future, because you have now conditioned players to look for used gear and given them a forum to do it.
2. The money made from selling the gear is just as likely to be spent on rent, booze, daily-life-stuff as anything else. And are we really supposed to believe that the players that sell their old kit at a car-boot-sale will then buy new gear at full retail from stores? More likely they will use the same car-boot-sale to upgrade themselves.
I'm not saying it's bad for the players. In the short-term its great for the players. At least until there are no more stores. What I am saying, is that it's not good for the industry. If you have a system that saves one set of people some money, that money has to come from somewhere else, right? That's how economics works..