Are you talking specifically about people joining up just to sell stuff?
The answer is simple - we (and you) have no way to vet scammers and people who do the wrong thing. On Ebay you have buyer histories and more importantly payment protection (to some extent) from Paypal. We have a new feedback system on SAU (which obviously did not exist when this thread was created), which offers some history on people, but of course it is buyer beware. You would not believe the number of scams that get reported to us of SAU users being ripped off - and not surprisingly it is almost always new users to SAU doing the scamming, and obviously SAU (and the buyer) have no real recourse.
By advising people to only buy from people that have been around on SAU for a while, have contributed to the community in some way (and even better if they have a positive feedback rating), is only prudent I would have thought.