You have to click on two separate pages to order an item. It helps if you actually read those pages before you agree to buy.
Contact your seller, aplogize and ask for a Mutual Cancellation. IF she agrees, and she is not required to, agree promptly to the cancellation and leave positive feedback "Thank you for helping a newbie." for example and Five Star Detailed Seller Ratings.
Your eBay account will not be affected by this.
If you don't want to do that, the seller will open an Unpaid Item Dispute. If you don't pay then, you will get a Strike on your buying account that will prevent you from bidding with sellers who have automatic Blocks against deadbeats. Which is most of them.
