I used to not, but I do buy empty cases now. Usually only if I am in person (like thrift or flea market) or in a lot on ebay, or if I need one for a game that is unruly to keep loose.
Then there are times I buy them on accident, as I had found from many years of buying boxed computer games... going through them now, sometimes they are missing some discs.
I'm loving you people that buy empty cases! I just made $50 on a box & manual for Castlevania today. No way I'd let go of the game itself, but for that much, you can have the box.
It's not to strange if your missing some stuff as a collector and you want the complete thing
Also dont forget that lose cases and manuals are pretty rare without the discs. Far more buyers than supply if it is a good game so people can be kinda desprate at times. Especially manuals can fetch nice amounts sometimes hundreds.
I would actually go as far as to say that completing a set in the lose way is more expensive than buying a cib copy unless you found one of the parts for a bargain but even than you won;t save to much if were fair. Only with very expensive games you'll pay more for the complete set than buying the parts lose.
That being said I would personally never sell a case and manual from any of my cib games doesnt look nice and as a collector kinda hurts, let alone from one of the best ps1 games ever made Castlevania SOTN shame on you