If you don't limit the marketplace in the current environment, you will attract the wrong type of members.
There's a reason we've seen most of the new members recently make a first post WTB. That's their first thought when getting approved, this is just a marketplace to them, that's the mentality of the collectard (TAKE THAT SKELLETARD).
Neo-geo.com has a certain rep, but whatever else it may be, it has taken seriously the defense of its forum from pure leechers and flippers who have no contribution outside the marketplace, banning decade long members with like 10 posts, all in the marketplace, and 100 feedback, for example.
How should the marketplace forums be limited? I don't know what hard and fast rule to apply. A post count limit would block hashiriya1, who has few posts, but is like the Santa Claus of import goodness, offering great prices on Japanese items quickly shipped from California.
At neo-geo.com, it's case by case basis, there's even a thread to nominate members to have marketplace privilege restricted. So it's a community effort.