Spot the crook?

I’ve beem thinking about how one can actually spot shady businesses. It’s not as easy as it initially seems—there are plenty of dodgy retailers that manage to look totally legitimate, and there are plenty of good companies that you might assume to be crooks because (for instance) they don’t list any kind of address online. For example, if you’ve ever shopped online for camera equipment, or browsed the ads in magazines, you’ve probably seen lots of stores in New York selling photo equipment at way below MSRP.