Try and keep dolls in a dry room away from dust and household traffic. Good to keep a ceiling fan running in the room where dolls are stored. If you must stack your dolls try either standing them straight up flat against each other or if you need to stack them from shelf up try and get the boxes to rest on the cardboard part of each box and dont let the cardboard rest on the plastic. It will damage the plastic. Try and store same weight boxes on same weight or lighter boxes.
Now for the looks of the boxes as you collect the dolls in their boxes, gently with a dry brand new paint brush with soft bristles go over the dolls box completely, this will get rid of any dust. On the day you receive them or sell them or at least once every few months take a glass cleaner cloth gently wet(like the windex already wet container) ....go over the box and the plastic being careful not to soak the box. Sometimes you may have to go over the plastic a few times because of stains but the boxes will look great when you are finished.
Do this before you send out any dolls you have sold. The boxes will look great.