I've only sold one camera out of the several I've owned, and it only had a thin strap that was hardly used. If I sell my DSLR, I would replace the strap with a new one before selling. Other than it looking dirty, the strap shows major wear marks where it rubs against the camera's lugs.
Same goes for buying a used camera, though currently I have no such plans at the moment (lust for latest and greatest, etc.).
