As somebody who just registered here, but has done a decent amount of research on keyboard in the past(and am probably ordering my first mech sometime this week), I can understand why you do not want to allow new users to the classifieds, particularly for selling or performing group buys, but as somebody who has been on SRK, they have a decent system in place to prevent abuse in the buying and selling areas and it works pretty well.
If you want to buy something, you may purchase it, regardless of user status, how long you have been with the site or how much you have posted. If you want to sell something you have to have 50 posts and have been active on the site for at least six months, so you've been able to build yourself a reputation before you may sell. Most scammers would never spend six months on a forum for that. Here on geekhack, where I know you do a lot of group buys, you could even take a step further, possibly requiring at least one previous transaction, or requiring to be a more well-established user(approved by a mod?) before you could organize one.
I think that would prevent a large amount of abuse, and still enable users to purchase boards from other members without having to spam for access. I know a few people that bought their first board here and pointed me here for some information when I first wanted to look into getting my own.
