Why? I can't remember the last time that someone got totally fucked by someone on this site, and then the seller tried to pop back online months/years later in order to try and scam someone else. Usually when someone completely screws someone else, they hide their heads in shame and never return to the site. For the most part, if the problem happens with a semi-regular user of the site, the transaction eventually gets taken care of.
The vast majority of bad transactions are first-time occurances that blindsided the buyer because they didn't see it coming. There's nothing I can do to prevent first-timers except for reminding people about the advice that's already posted in the for sale forum - use common sense, use a method of payment that has some sort of recovery option (eg: paypal, money order), etc. The iTrader system has been provided so you can see who has followed through with their transactions and got positive feedback from the buyers.