I used flippa once and it was terrible. I would NEVER use them again.
I sold a website to some person who didnt end up completing, When the website was sold I had to pay $500 to get the contact details of the buyer, when I did this the buyer didnt answer emails or anything, so I contacted Flippa for a refund many times, they did not answer any of my emails. Fortunately I paid with PayPal and I filed a complaint, Flippa didnt respond to PP either, eventually my case timed out and I got my $500 back.
I lost my other listing fees though etc...
Total cowboys if you ask me. Stay clear.