And how it supposed to work? Who should ensure that those mandatory previews would be really uploaded? If you're thinking about moderated approval, then forget it - that would require either huge human resources or approval times would be so long, that will effectivelly would kill new uploads. Most likely it would be both. The only way to fight piracy with some efficiency is vigilance of sellers. Cgtrader should have great interest in fighting piracy and should introduce some tangible reward system for those who actively reports IP infrigements. Usually it's not that hard to spot pirated models and i can tell you amount of suspicious models in this marketplace is huge. Unfortunatelly, to gather enough evidence about each case of IP infrigement takes time and effort and without any kind of reward for vigilant actions, you sooner rather than later will loose interest in doing so.
Without willingness of CGT to declare war against piracy, you can do nothing about this. So sit back and relax, you get to used to this, as cynical as this may sound.