I too have this happen quite often, I try to source from alternative supplier but if I can't, then I refund, humbly apologise and cancel transaction.
Stupid story, sold this item, my supplier was out of stock, found another supplier, so, rang them, told them, hey, I usualy go through this company but they are out of stock, do you have any, they then made a quick phone call and come back and said no, they are also out of stock.
Talking to my supplier, they stated that the other company had rang them seeing if they had stock. apparently the second mob only sell like me but through a website.