That makes newcomers a juicy target for competitors to leave fake negative reviews. If you need to pony up $10000 to start a vendor account, a competitor can spend $1000 to leave 40 bad reviews, assuming $50 each and reselling at a 50% loss.

If you need a 90% positive rating to be trusted, you'll need 400 good reviews, about $20000 in turnover.

It's too cheap to ruin a vendor and too expensive to maintain one.