You'll be asked to leave feedback for your transaction seven (7) days after the auction or private sale is declared won. This feedback invitation expires after 60 days, so you can wait until both money and website ownership have been exchanged to review the transaction.
Buyers will receive a second feedback invitation 3 months after their purchase. This time, they'll be asked to review the site they purchased. Did it live up to the seller's claims? Were there any hidden issues with the site?
Site quality feedback invitations expire after 30 days.
Be sure to look at both kinds of feedback when evaluating a seller.
More on Flippa's feedback policy can be found here.
UPDATED: May 17, 2018