There's two sides to this. Obviously most people would prefer that people become members and contribute first, before selling stuff. But on the other hand, these people might be providing good deals that the rest of us might want to take advantage of, regardless if they just showed up to sell. Obviously CraigsList is a good example, there are thousands of Colorado reefers that never even go onto forums like this. My two cents.
As for the post above mine, I wouldn't want to be a part of any online forum that actually "voted" on new members. That's taking it way too far, imo. We're all adults here I think, not a fraternity, lol.