Either get a box specifically made for shipping a long gun (your local gun shop may have one they will give you) or make one using a box scrounged for the purpose (any box used to ship long items like blinds, curtain rods, etc.). Make sure to pad extra well with more cardboard, foam rubber, etc. It is always a good idea to cut extra pieces of cardboard to line the ends of the box with so the butt or barrel don't poke through if/when the box gets mishandled.
Or you can buy an inexpensive plastic gun case and use that with the request that the buyer mail the empty case back to you. I have done all these with no problem.
The CMP shipped their rifles in really nice strong cardboard boxes with foam rubber padding. When I sold some rifles, I used the same CMP box many times, always asking the buyer to return my box. They always did.
Shotgun has to be unloaded of course. If it breaks down, I would do so and ship it like that. Whenever I shipped a rifle, I always removed the bolt, wrapped it in foam rubber, and packed it separately in the box.