@abrac, I wasn't sure if the author of the review was accurate in his description. @garmonbozia thanks for the clarification! I was unfamiliar with the root word; that makes much more sense to me now. (Aside: are you excited about maybe more Twin Peaks??)
I think it's a pretty good explanation, as is garmonbozia's. But I'd add a couple of things. I agree with garmonbozia he's not so much a salesman but a peddler or hawker. In his left hand it looks like a small megaphone for calling out to people as they pass buy. He might have a horse or cart though not shown.
The Italian bagatella means "a trifle." So he's someone of little standing and might be a bit shady. His merchandise is probably cheap and not very good. It's easy to see how this image would have evolved from the earlier magician images since no doubt he's familiar with the art of trickery.
Here's a link to a long paper on the etymology of bagatella. Probably more than you'll ever want to know but has some good stuff.