The watch list is there for buyers to put items they are interested in until they decide whethere to buy or not.  Once they press the buy button, they are committed and they have removed your item from sale so no one else can purcahse it.  Even B+M stores don't hold an item while you decide whether you want to pay for it or not.

If the buyer, for whatever reason, has decided not to procede with the purchase and they contact you, you offer them a cancellation they must respond to within a set time frame (because you are a nice person and offer good customer service).  If they don't relpy or you don't hear from them at all, start a NPB claim to get your fees back.  Relist , move on, and smile.  🙂