The act of putting an item in the Shopping Cart is simply that: putting an item in the cart. It doesn't mean you have commit to buy until you Commit to Buy or Request Total From Seller. If that happened, and the seller is reluctant to cancel the order at your request, you may either ignore the directions to pay and receive an Unpaid Item Strike on your account, or pay for it and return it when you get it. Providing the seller allows returns.
If you bid on an item and then won it, you've bought it. There is no turning back from submitting a bid, that's why you get a second page to confirm your commitment to bid before the bid is applied.
Ebay is usually kind about new buyers and their first Unpaid Item Strike (UIS) so you may have success with getting it removed if you call Customer Service and explain things. Too many UIS and you'll find it difficult to conduct business on ebay as most sellers have their systems set to block any buyer with more than two unpaid item strikes per year.
In future, be sure what you're buying is what you want before bidding or hitting that Commit to Buy. Everyone makes mistakes but too many and you'll find your account hindered which is a bummer.
Oh, and you can't contact the seller by replying to the system-generated emails that ebay sends. You have to go to the Order History and Contact Seller via eBay Messages.