The last thing you or your eBay buyer wants is for them to accidentally throw away part of their order or believe they received an incomplete order (it’s painful to even read about, right?). That’s why whenever you pack LEGO orders using cardboard or any non-transparent packing material, make sure you label the item inside.
Multiple pieces packed in one box (per LEGO manufacturing)
Some minifig capes or rubber belts are packaged two to a box. A buyer may assume the pieces are only packed one to box, though. Labeling the box or including a note saying “two capes in each box” can help prevent a confused or angry email from a buyer who thinks they received an incomplete order.
LEGO instructions, stickers, minifig capes or fragile packed pieces in cardboard
Sandwiching fragile LEGO pieces and LEGO paper products between two pieces of cardboard helps prevent creasing and damage during mailing. Unfortunately some buyers will mistake your safe cardboard packing for trash! Yes, that means a $150 Millennium Falcon UCS sticker sheet could get tossed in the garbage bin. Ouch! Labeling the outside of the cardboard with “Sticker Sheet” can prevent these painful and costly “throwing out the the baby with the bathwater” blunders.
Further reading:
What’s the damage?: How to Identify and Prevent LEGO Piece Deterioration (BrickingTips.com article)