I have been collecting only excellent condition air pistols for the the last 20 years and have have lost count of the the number of good pistols I have seen at arms fairs etc. that have been ruined by rust on the right hand side from being stored in cardboard boxes.
I think moisture from the place they have been stored in is retained in the cardboard which is in direct contact with the side of the gun thus causing rust and pitting.
I keep my pistols in a display cabinet known to be completely dry and the boxes in a separate dry place but with a note inside the boxes with the model and serial number of the pistol contained in the box so they dont get separated for future owners.