I bought a Voucher from Groupon, Had a horrible experience with the seller, who refused to honor the voucher. I was out the voucher money plus a deposit I had paid the seller for the service. I tried to leave a rating on the site and it was not possible as the seller hadn't redeemed my voucher.
Only when the Merchant themselves redeem the voucher can you leave a rating. Therefore only the customers that have received good service leave ratings. These ratings CAN'T be trusted.
P. S - Groupon replied to me completely ignoring their misleading rating system which is tricking customers by avoiding bad reviews and solely focusing on my bad experience with the merchant.