Although I gave up selling on eBay some years ago, you could try doing what I used to do. Get a decent pen that writes clearly, write the buyer's address in full on the package and then go down to the Post Office to get the postage label sorted out and paid for, whether you choose to send the item via First Class, Second Class or, in the case of sending items to overseas buyers, International Postage. Once you've done that just stick the postage label to the item, make a note of the tracking number and upload it next to the buyer's item when you're marking it as having been despatched.
Finally, before you post the item out, make sure that you write on the reverse of the package "If undelivered please return to..." followed by your name and address so that you get the item back again in the event that the item cannot be delivered for some reason, such as if the Post Office cannot find any record of that address and/or postcode.
