I also flatten only the boxes with peanuts, and save the boxes with molded inserts in their entireity. Although they take up a lot of room we have a crawl space that we can use. I put them in larger cardboard boxes (about 20/box) and store them. My reason for keeping the inserts is not that I ever intend to sell the collection, although I have some duplicates that may go, but should we ever move I want to insure their safe delivery. However, I have received a number of pieces through the mail that are packed in peanuts and have no damage.

If you are pressed for space, and don't care about "original packaging" then the best approach is to throw away the inserts, put them in a nice rubbermaid tub, and store them in a safe dry space. If you need to repack buy a bag of peanuts and repackage. Of course you keep the COA and other certifications to accompany the pieces. Also I don't use the tent cards, but find it a lot more interesting to have people point at a piece and be able to identify it.