Well the separate usergroup is just that, a usergroup which isn't defined by post count or anything like that, each member will have to be assigned it manually. Anyone else, won't be able to see the forum.
We would however need to have some form of criteria. I don't want people ranting and raving because they didn't get a spot in the group (yet) and kicking up a fuss, so we'd probably need a voting system or something. If the majority trust you, you get access. If not, no moaning.
Payment and postage would be a bit tricky. I want no responsibility for this, so we'd have to keep it as tigh as a duck's arsehole. Payment would either have to be Paypal Gift (non reversable) or bank transfer. In either case, a screenshot would have to be saved to a database here on the site in the event of goods not arriving, arriving broken, etc etc. Same with postage. Only tracked services would be used, and a tracking number would have to be supplied with the transaction.
We'd need a process for this however, and time limits. So say someone shows interest, the item is reserved for three days. If payment isn't made within three days, the item can be relisted for sale. If however the payment is made, then the seller will have three days to send the item and provide a valid tracking number. We then need to allow some time for shipping, I don't know how long shipping takes in the US but it's usually next day here with Royal Mail, so I'd allow 5 working days for if something goes wrong.
Buyer says he / she received item and is satisfied, seller has received cash and is satisfied, job done, thread locked.
EDIT: Of course anyone partaking in this will have to be aware that the forum or its staff members assume no responsibility for the action of its members. The most we can do is ban members, if you wish to pursue non payment or fraudulent activity, you're on your own. I realise we're all mates here and the odds of this happening are slim to zero, but there's always the future, and our Google ranking has been pretty kind lately.
I'm considering a trust system, so that members can leave feedback for each other but let's get the basics sorted first.
Anyone see any loopholes / issues with this? Any ideas or suggestions?