Well if its a box for one of the great classics, such as say: 6285 Black Seas Barracuda then its probably worth keeping.

You can still do the sealable plastic bag option, you just seal up all the parts and chuck the bags inside the box. It's easier to check if all the bags are still inside the box than scouring the bottom of your bag for little parts.

Sellability factors (to me anyway) are ranked as such:
Completeness>Condition of Parts>Shipping Cost>Instructions>Box