You're right, 'suly', you must ask the seller to cancel. So when you are sent a request to cancel in your ebay messages, be sure to click to respond and click to accept. This way, the seller gets her fees back, and you are off the hook and it's all over in an instant.
The seller does not have to cancel, though, and can choose instead to file an Unpaid case against you. At that point you either pay up or get a strike. A second strike and it is just about impossible to bid or buy on ebay for the next 12 months, as sellers block these nonpayers. Mistakes happen, but be sure it doesn't happen often. Have a look at the following link about Unpaids:
http://pages.ebay.ca/help/buy/questions/unpaid-item.html
Most sellers, however, will send the cancellation to let you out right away, - if only because it is quicker and easier than going the Unpaid route. So be sure to speak kindly to your seller, and thank her afterward.
Good luck. 🙂
