The quickest answer I feel I should give to this question is this: not your handwriting! Scrawling addresses on boxes with a pen is extremely amateurish, and you shouldnt do it. Print your own labels instead. But what should you put on them?
The buyers address: This might sound obvious, but you need to make sure the full address is there, including country and zip or postal code. Make sure you spell their name correctly, too, as they might be a little insulted if you dont.
The eBay item number: Write this somewhere discreet and quite small, in this form: Item number: 123456789. Under the address is a good place. This makes it easy for the buyer to find the auction again if they need to look at it for any reason.
Your logo: Putting your logo on the shipping box looks professional, and lets your buyers see what it is that has arrived. A good logo can make the whole package look very nice, and can help build recognition of your business.
Your address: Its worth putting your own address on the box in case the item cant be delivered and needs to be returned to you. It looks good to write this under your logo, as it reinforces the impression that youre a...