Bubblewrap - I use a lot of it, as people who've received basses from me will testify to
I start with a good box, and you can normally have one from your local music shop if you ask nicely. Use heavy duty tape to not only seal it, but support any weak spots (e.g. corners, etc). Remember, critically, to make which way up you expect the item to be stacked - neck first will not be healthy!
In terms of the bass, communicate with the buyer - make your clear your plans and get their agreement; especially important if you're doing things like taking the neck off. Bubble wrap and card will support sensitive areas nicely.
Also think about how the bass will move in the case - will parts rub, like tuning pegs, strap knobs, etc, and adjust your packing to cover. I don't know if people collect old shopping bags when they're done at Tescos, but scrunched up they're useful to 'pack out' the box and secure the bass - alongside styrofoam, shredded card, etc.
And, thinking out loud, loosen the strings!