Interesting...I was thinking about putting one of my sites for sale on there that makes around $3000/month on autopilot. I figure that it is worth somewhere around what you sold your site for. But as far as the payment and dealing with bidders I am unsure how to go about it. How were they supposed to pay you via paypal? Is that safe couldn't they do a dispute or paypal lock up your account for having $24k deposited at once. What was your expected payment method?
This has happened to me before. Tell Flippa, they will waive your fees and get it re-listed. I have actually had the winning bidder not pay and then when it was re-listed I sold the site for more.