Go to your Purchase History.
There is a drop-down menu. Pick the option that allows you to Contact Seller.
Ask for a Cancellation. Cite the item number or title of the item.
Be polite. The seller technically isn't obligated to accept your Cancel Order request, so you don't want to antagonize them.
If this goes to Unpaid Item action, so be it. You're allowed one boo-boo and can probably have it wiped from your account if you call Ebay Customer Service. (This is the part where being polite can save you time in the long run.) Likewise, if your seller starts behaving in an impolite manner, you can report them to Customer Service for unprofessional behaviour. This is time-consuming and not something I would advise doing unless things get out-of-hand. We will assume this doesn't come to that.
Good luck.
Oh, and make sure you examine all available information before your next purchase. It helps to sit down at a desktop computer to do that, to be honest. You can see more information at once that way, than on a mobile device.
