I have sold approx. 45 basses and posted most of them. No hard cases, unless the bass came in one, which has only been a couple of times. Lost none.
Bomb-proof packaging is the key. The bass gets put in a gig bag most of the time, and usually double boxed. I have frequently fashioned my own outer boxes from former bicycle boxes - Halfords has always been keen to give them away when I've asked.
My method is to make sure the bass is surrounded in cushioning material (bubble wrap, foam, polystyrene, balled up paper - whatever I have to hand) on all sides and the box is fully packed with it until there is no movement inside the box. Not overpacked to the point where the box might burst, just packed enough that it takes a little effort to close it. The whole thing must move as one when shaken.
It takes time and effort, but it's worth it for a safe journey to its new owner.