It seems more complicated than that - Publishers (representing more authors), pseudonyms (and pluri-pseudonyms) etc. - but not dramatically so. A battery of people (human level intelligence) will check to validate the "upload" process, and automation aided.
Already now: what checks that John is not selling Jack's work? The mechanism is similar.
Do tell. In the gigantic ocean that is Amazon's marketplace, what do you suppose a single intern could do that would not be immediately circumvented?