Occasionally I get buyers send me their delivery address before payment, then after a few days and I haven't posted, they message to say they don't want it anymore. Not sure if they're ust chancing it or the reasons they give for canceling are genuine and they just got ahead of themselves. Still, the advert doesn't get changed to 'sold'until the cash is in.
On the whole though, BC marketplace is a good place to buy/sell, everyone seems genuine and i only go ebay etc if I have to.