One thing I like to see when I get a game is sometimes a seller will add a small peice of bubblewrap or cardboard inside a game case(dvd or cd) so that the disc doesn't pop loose and rumble around inside the case during shipping.
Another thing is to make sure you use some good clear packing tape on the label. Make sure you cover the destination address and return address, but do not put tape over the barcodes they need to scan. Also print out the item you sold from your listing and put that in the box along with a few small pieces of paper with your return address and destination address mixed in. Should something happen to the label you'll have back up info inside the box. Should the box end up damaged or opened, they'll still be able to deliver the package.
My mother used to sell a lot on ebay and worked for the post office, these are things she did to protect outgoing packages.
Good tips! I've def liked it when a little piece of bubble wrap is added where the little release button is inside the case (that way, if there's any pressure, no damage to the artwork or manual will occur).
