Glad Mica weighed in on this one. Especially since she's not slamming the door closed on the idea. A couple thoughts in the event that there is any sort of a formal move to this idea.
One forum that I am associated with only allows Supporting Members to post for sale ads. Supporting membership costs $20/yr. then you don't have to care whether the seller ever posts again. Buyers do not have to be Supporting Members, but sellers do.
Another forum requires a member have a certain minimum number of posts before they are eligible to post gear for sale. Like Alembic Club, that forum is also company sponsored by a family run instrument builder.
As far as posting used Alembics for sale hurting Alembic's business? I think the most likely buyer of a new Alembic is the owner of a used Alembic. A used Spoiler was my gateway drug. If this forum was around when I was in the market for my first Alembic, I would have had a better selection of used instruments to choose from and would probably have gotten into the game earlier. If I had purchased it from a dealer listing a used instrument on the Alembic Club, I would probably be more predisposed to buying new from that same dealer.
Peace,
James