Before dc and before tweaks in the etsy system....people - customers were able to click and cart and tie up your items for up 7 days. While you waited for the reply, or a message from the seller indicating how they were going to pay for that item.
I can't remember when the change came, probably the end of 2008, but it was fixed. And the item wouldn't appear as sold until the customer paid in full via PP.
Perhaps their a situation with the payment or credit card at the customer's end? I don't use dc, but I've heard good things from other sellers about the process, then I've read many like your situation today. Good Luck hope all works out.