Well, concerning the flakers - there are times when someone commits to a deal with you and says to expect funds shortly. Therefore, you turn down other offers to buy from legitimate buyers who have the money and are willing to pay. After waiting and waiting, you find out that the "buyer" changed his mind, so you techically lost potential sales to others. In that case, I agree with leaving negative feedback to buyers.
As fas as I'm concerned, whether you have the cash in hand or not, if you commited to buying an item from a seller, then you have an agreement and should honour it. It's the same with ebay. If you commit to buy, then you have a "contract" with the seller and are expected to honour it.