First, Flippa takes its commission as soon as the sale is marked as completed. But if a buyer doesn't go through to the end via Escrow, or requests a chargeback on Paypal, the Flippa still holds on to their fee.
Second, sellers can hide abusive or hurtful (wrong, malicious, etc.) comments, but Flippa admins un-hide them before the sale is made. Therefore, potential buyers can get a wrong impression of the listing, even if the comments are false.
Last but not least, there are many, many, many scammers on Flippa, trying to sell sites with falsified data.
Overall, I'm never gonna use Flippa again. Ever.