You raise some interesting points. A lot can be said about the idea of moderators catching and not approving clips that do not meet minimum levels of quality. In fact, you would be surprised to see how much stuff is not approved for this very reason. It's one of the things I think that sets Curvage apart. Every clip gets manual approval by a human!
However, it's important to remember, models here are not employees of Curvage. We operate a website that they use to sell content and as such we walk a fine line. We cannot tell them HOW to work, or WHAT to produce. If the clip plays with enough quality and a person could responsibly enjoy it plus it meets all the other guidelines, then it would likely be approved.
Consider also, that many models are learning as they go and over time their quality for video and editing improves. They learn what the customer wants to see. They cannot gain this experience if they are blocked from doing so.
If a model consistently creates content that does not meet a customers expectations, then customers will migrate their business to other models.
Rest assured though, that clips which are egregiously bad or contain other issues are certainly blocked from sale.
Some examples of things we would block
Clips that don't play or otherwise glitch
Clips that are < 2 min long
Clips with obvious and egregious nonsense like a giant part of the video has a black circle blocking out everything good.
Clips that are duplicated
Clips that are misrepresented in the screen shots or title.