It's been a while since I used eBay for anything at all, let alone selling parts. However, when I did, I used the postal service for my shipping. They have a variety of box sizes available and the rates are reasonable.
Make sure the description indicates that the bid winner will pay shipping. I also had folks pre-pay though Pay-Pal for the item, then ship. Do not entertain any offers to go around the system, they are dishonest people trying to short sheet you. Photos and a good description of the item help enormously, but if you are trying to sell an item that no-one is interested in, like the pass side C-Clip from a D30, then eBay will just end up with their fee, which went up as I understand, and you will be left out.
-Ron