That depends upon what listing you want cancel.
If the listing is sold and your buyer has paid, being "unhappy" with eBay is not a valid reason to cancel.
If the listing is sold and your buyer has not paid, you should follow the Unpaid Item process rather than canceling.
If the listing is sold and your buyer has asked you to cancel, you can simply find that listing in your All Orders page and end it.
If it is a fixed price listing that has not sold, you can simply find that listing in your Active Listings and end it.
If it is an auction with no bids, you can simply find that listing in your Active Listings and end it.
If it is an auction with bids, you may need to cancel those bids first.
