The owner and quite a few members at http://experienced-people.net/forums/forum.php
have quite a bit of experience buying and selling sites, including using Flippa to do it. I would try to gather information from there as well if I were you.
I've only used Flippa to buy a site. Not that Flippa did much for the process.
One thing to be aware of is bidders that will never actually follow through with the payment. I've seen lots of auctions that ended with a winner, only to be re-listed because the buyer stopped replying, kept postponing payment etc.