As an ordinary buyer, all you can do is leave appropriate feedback, and refuse to accept a cancellation request if the seller sends you one.
EBay itself will penalise a seller for making seller-initiated cancellation or refund, unless the buyer has accepted this. They receive no return of fees paid for the sale, and a defect is placed on their account. It only takes a small number of defects for a seller to be permanently banned from selling.
Despite all the rules and penalties, sellers who won't sell and buyers who won't pay continue to be the bane of eBay.
