Now, there's a large body of literature saying "We built this software, a group came and used it, and they began to exhibit behaviors that surprised us enormously, so we've gone and documented these behaviors." Over and over and over again this pattern comes up.
The obvious response to someone keenly highlighting the flaws in such self-regulation is "what else is there?". Who's to say EBay won't turn around and abuse this new power of secret feedback to start taking bribes for favoured suppliers?
If you say that what stops them is that they'll go out of business, defeated by a more honest rival, you're simply shifting faith in one bottom-up system (EBay's reputation) for another (the market).
If you say that it's government oversight, how and where is that being expressed?
The problem with labelling human self-regulation "weak" and calling for leviathan to come and sort it out is that leviathan is just another flawed human being, and giving him more power, simply creates a greater temptation to abuse.