Regarding the reviews, I haven't been on these sites but I suppose that they would only allow buyers to make a review. Even if sellers cheat and create fake transactions to be allowed a review, people who are not happy with the service will still leave a bad review. So because reviews are not under the control of the seller, there is a high incentive for them to be honest.

Don't think of it as yelp, think of it as Alibaba/AliExpress.

Sellers can always cheat, but it's harder and the risks of being booted off the site if you are a crappy vendor are higher as well.