I sold a Ruger PC Carbine with a cardboard box, and brought it for the sale, they threw it away right in front of me after taking the length-of-pull-shims/extras out...
I sold a Beretta M9 (to upgrade to the 92FS INOX), Ruger SR-9, MK-III 22/45, and a MK-III, and they all wanted the plastic cases/boxes...
It apparently depends on the buyer... and I was happy I got the box/case for the INOX 92FS