The idea of making the forum private to members is about community building, not preventing arbitrage, though. A lot of posters come here to share deals with a community of like-minded individuals. Registration nudges towards more active participation, but doesn't force it. Trying to lock out resellers is a cat-and-mouse game, and also the community might not even want that. Like others have mentioned there are some users here who do reselling on the side but still actively participate in the community. And like you folks said, there are other public places for arbitragers to shift to if they don't want to register here. What's the downside of that happening?
At any rate, I still think it's a good idea. It won't magically solve all the problems, but could inch us towards a tighter-knit community.
EDIT: Also, I am sorry for bringing this up on this thread, potentially derailing it. I am not even sure that resellers "killed" this deal: I only grabbed 4 because I am not made of money, but even for a home NAS it's not inconceivable that a lot of us here are running with at least 12 or 16 disks. The folks who said they bought 24, I am not immediately thinking will be reselling them.