These sorts of ads are ubiquitous in the early days of computing and video game hobbyist publications. Many of them were basic wholesalers who got a discount for buying direct from the publisher in bulk, then did a slight price markup which still beat the prices at the computer shop. There were fly-by-night operations, but they often didn't last very long. Since early adopters of home computer tech tended to be a more hardcore user base than what we find today, outfits that screwed over customers quickly found their exploits broadcast over every public, semi-private, and private BBS service in the country and watched their sales dry up overnight through word of mouth. :)
Interestingly enough, a good way to avoid the scams was to use companies like this one, which allowed C.O.D. (charge on delivery) ordering. There was often a small charge for this service, but it meant unless their product arrived safely and satisfactorily to the customer, the business didn't get their money. :)
I too love these old ads, as I grew up at a time when you'd still see them in the back of EGM and GamePro. :)
It seemed like by the time the late 80s rolled around, things had settled down. There were still plenty of ads like this but they were mostly from the same half dozen companies or so. I remember I ordered Pool of Radiance for my Commodore 64 from one of these places though I can't remember for sure which one...