I've never sold on ebay, only bought, and I've only bought books, scale models, and golf equipment so my advice is extremely focused on those things.
For scale models, shrinkwrapped boxes are nice because it implies that the kit is in its own little time capsule from the factory, but I prefer boxes that have been opened so that the photo can show what comes in the box, with a nice photo of the decals (the first part of a kit to go bad), and whether anything is broken, painted, or missing. If you buy a kit at a garage sale and intend to resell it, I suggest you take the time to (a) verify that all the parts are there and (b) put the pieces in a ziploc bag. Put the decals in a separate ziploc bag to keep the moisture off of them and to keep them from getting scratched.
I also prefer buying kits in groups: i.e. 2 kits up for auction together, or a kit with a book, or something along those lines, to get the feeling that I'm getting a little something extra for my shipping trouble. If you put stuff up for auction individually, offer to combine shipping.
For books, mention if the binding is tight, are there any color photos, worn edges on the cover, # pages, copyright date of the edition, stains or discoloring, and, believe it or not, whether it smells or not. Books from a smoking home can really stink up a small room.
For golf clubs, mention scratches on the head or shaft, wear on the grip, shaft flex, stickers/labels, grass stains, and such.
I'm willing to pay a bit more for the convenience of using PayPal than I am to cut a personal check or, even more of a hassle, a money order or cashier's check.