Please read Lillie Yifu's blog entry, "Triangle of Fire." It's eye-opening.
I guess I was extremely lucky when I came to SL, or perhaps I was/am blissfully ignorant of the extent to which I may have been fleeced.
So... how to stop the scams? It's going to require a trustworthy source of inforrmation—a way to ask whether a vendor provides reasonable value for L$, or a way to search for something and have the crooks filtered out of the search result.
User ratings leap to mind—but can those be gamed, with scammers buying good ratings or creating myriad alts to give good ratings? Another possibility would be an organization that functions like Underwriters Laboratories, but for SL. I don't have an answer, but one will need to be found.