Well, you had to click TWICE to buy the item.
The seller is the only one who can cancel the item.
If she is in a good mood, she will be sending you a Request for a Mutual Cancellation. Agree promptly and you will both be released from the transaction. Leave positive feedback like "Thank you for your help with this."
If she is grumpy today, she will open an Unpaid Item Dispute. If you don't pay up, you will get a Strike on your buying account which will make it harder to bid on eBay, since most sellers have automatic Blocks set up against bidders who don't pay.
