If I order on eBay and then cancel, and the seller didn’t respond after five hours, what can I do?

That depends: a) if you made an auction bid, you can simply retract your bid. b) If you actually bought an item, requested a cancelation and the seller did not reply - don't pay. The worst thing that can happen is that the seller opens an unpaid item dispute against you, he wins, and you get a strike against your account (which can be removed if this is your first offense. Keep repeating this action, however, and sellers (or eBay itself, for that matter) will start blocking you from buying or bidding). Remember that 5 hours is way too little for a response - you should give the seller at least 24 hours to respond, and longer on weekends. On the other hand, if you have any reason to believe that there is some kind of fraud involved in this selling, click on the report button at the bottom of the page.  Again, you should pursue this course only when all other options fail: buying and not paying can get your reputation burned very quickly on eBay.