If you have bought something and want to cancel, I don't think you can do it yourself. You will have to ask the seller and agree to cancel when they send the form for it.
Or the seller might be annoyed and decide to file an UNPAID ITEM case against you to get their eBay fees back. To get the most out of eBay it is probably a good idea to register. Its really easy and doesn't require much.
To view items as a guest eBay says this:
The policy page states:
You can buy some items without registering as an eBay member, by using Buy It Now, and you can see your order details by following the link in your confirmation email. On the order details page you can see your purchased items, shipping status, and tracking numbers, return an item or report that you didn't receive an item.
http://pages.ebay.ca/help/buy/buying-as-guest.html
How to register:
http://pages.ebay.ca/help/account/how-to-register.html#steps
