I've never had any problems buying from forum members....my purchases have all been well-communicated, shipped in a timely fashion, and condition of items have been as-described. Same with selling items, fast payment, etc. That said, there seem to be a few problems here and there.
Not sure if seller/buyer ratings would help too much, I haven't seen very many, but fraudulent sellers seem to disappear after a sale-gone-bad. Buyer ratings seem unnecessary, payment should be received prior to shipping.
Plus, buyer/seller ratings would introduce yet another element of conflict if a sale is disputed on questionable terms. I seem to remember a few well-documented cases that could have/should have been resolved PRIOR to consummating a deal. Exchanging detailed pictures, etc.
Just my .02.