You have uploaded the listing, but it has not yet 'indexed' and cannot be seen by the general public?
That gives you a few hours.
Go to your My eBay-> Selling-> Active
Find the listing you want to cancel. It may not be there for a little while.
Look on the right side just above the Description. There is the listing number.
Copy the number.
Go to the Site Map. Go to the centre column and look for Cancel Listings.
Click and follow the instructions, which include pasting in that copied number.
