Aside from that the article seems to completely fail to take notice of the fact that Amazon enjoys a rather extensive monopoly across most of it's operation. Of course the seller will always have more information than you, but if there's a large selection of sellers to choose from, then there's zero incentive for them to use that information _against_ me or to prevent me from having it.
Consumers don't lack savvy, the marketplace lacks options.
Not sure if it's a perfect analogy, but if we compare the Amazon marketplace to a giant flea market, I think the claim that consumers need a reasonably high level of 'savvy' to operate effectively is pretty easy to defend. When there are lots of sellers, lots of noise, and difficulty verifying quality, the consumer will frequently lose out despite the surplus of marketplace options.
Your own example proves the point. Flea markets might be cheap to run for the owner, but would people go there by choice if there were a superior alternative?
Well, it depends. Walmart's quarterly revenue is still higher, though they don't have quite as broad a selection of merchandise.