"Does cancelling n order because you are out of stock count as a defect?"
Yes, selling something you don't have is one of the seven deadly sins on ebay. You have been on ebay for fifteen years and don't know that? There are better workarounds than cancelling a paid for sale in your situation.
First of all you can contact the buyer and explain that you are looking for the item as it seems to be misplaced. Then go order the item from another seller and forward it to the buyer once it arrives. I am in the middle of doing this at the moment and sent a similar alternate item to the buyer I did have in stock and explained that they would be receiving the correct item at no cost in the near future.
In ten years selling here this happens once in a while when you juggle 60,000 inventory items. It also seems that ebay's software makes the mistake of letting us relist a sold item when we relist in bulk. Drives me nuts but many times I just drive to a supplier and find the same item to mail within 24 hours.
The fastest way to reach ebay is to ask for a call back at 2:20 a.m. Works for me every time as they call within about two minutes or less.
