It is impossible to flake on someone in my opinion. Until you have the money in your bank account, the item is still for sale. Its dumb to have bidding and is against the rules but I have always worked on a first to pay gets the goods system, as a result its impossible to flake on me unless you ask for a refund before I ship.
I have had people threaten to leave me negative feedback for flaking when all I asked for was a shipping estimate and then said no thank you. It can get ridiculous.