I'm still relatively new to the forum, but in the last few years I've probably had 5-10 transactions made via buy/sell and have had no problems. For the most part you can tell a sketchy individual within the first PM or two (if not in the ad itself).
I have bought and sold a lot via forums, Craigslist, and eBay, and as I see it the largest advantages to dealing in forums are that for the most part gear is priced at what it is worth and that people are relatively knowledgable, honest, and human. While there is the occasional (general intentional) great deal or someone looking only to profit, this forum has been for me the epitome of the advantages of forum marketplaces.
I see no reason from a usability standpoint why GetDPI should implement a fee to buy and sell.
Also, I am currently a student (photojournalism), and even a $10 or $20 fee is considerable and would have given me serious pause before beginning to participate in the buy/sell forum. If there is ever any thought to offer some kind of student discount in the Gallery Member Prints For Sale area I would definitely be interested.