This is a good point, but personally I don't think a post minimum is beneficial.
I joined this forum to buy my dream bass and having acquired it, stuck around. Everyone has to start somewhere and I think the buy/sell section is a great because you can start with a focus. Admittedly using this place once to sell an item is abusing it a bit, but it's the perfect way of dodging evilBay and finding your target market. I love how active the market is on this forum, it gives me a chance to pretend I could afford some of these beautiful basses.
I know I don't post much, but I'm on here everyday reading what all these wonderful people have to say and occasionally feel a need to throw in my two cents (like now). I think a post minimum would deter new users, I know it would have deterred me. Plus, it'll will more work for our amazing moderators (suck up) to filter out all the spam posts.
So new users come and go, but at least people are coming and going and surely that's the sign of a healthy forum. I'd hate for this to become a clique instead of a community.