Apparently users only get banned from Dan.com if they are repeatedly abusing the system. So no real penalty for a non-paying buyer unfortunately.
I have another one going through (well not going through) at the moment and the buyer has gone dark - he has already been named and shamed on the forum as a messer but sadly I didn't see it before accepting his offer, so a time wasters section could prove useful.
Yes, fake offers happen quite a lot on DAN.
In exchange for a very simple process (no account setup and minimal validation) which can help facilitate a sale, you get the risk of dodgy offers or even ones made by mistake.
I had one nice offer for a domain but never completed the payment. After the guys at DAN followed up with the buyer, she said she thought she was bidding on a car.
Yes, it was a car related domain, but still...