Sellers sometimes do drafts and make corrections on a listing which might create another listing by mistake.
Just by adding a "." to the title to correct a listing might create two listings.
If you have a 'sold' item but don't have it...contact the buyer and let them know of the situation ASAP.
Offer the buyer something in place of the sold item...maybe a little more expensive.
I wouldn't do an out of stock thing...and get a 'ding'.
Most buyers would be happy to get a better deal .
Would also save a nasty feedback response from the buyer.
eBay prefers the seller and buyer work out the situation.
