If it is lettermail/untracked, I would cancel due to a problem with address. I would send them a polite message explaining that Canada Post's Address lookup stated their address was incomplete. I would then invite them to re-purchase it if they would like. The problem with untracked is that you are on the hook if anything goes wrong. With tracked, you could at least give them the benefit of the doubt because there are ways to redirect or retrieve the package for them if they later contact you.
A guest account is less likely to respond to messages. You could see if they have a phone number attached to the transaction. If you go to your orders page and select Print Packing Slip from the dropdown, the packing slip will have their phone number under the address. You could call or text them to inquire about their address. Keep in mind, some people view eBay as more of a peer-to-peer type of marketplace and may not even be aware you have their phone number. They may not like the idea of a random eBay seller "from the internet" calling them, even if there is a reason for it.
