They want to avoid bags going to the auction house (vendor, depths of the shrines) with stuff in. Not allowing them to be sold is a simple way to keep that from happening.
So only allow them to be sold with zero contents. This is not complicated. Other games allow the buying and selling of empty bags, so it's expected that you can get 700 currants back when you go to upgrade from 10 to 16. Discovering you can't sell the 10-slot bags is a rude surprise. If nothing else, the item needs to explicitly say that unlike anything else in the game you cannot sell it back to a vendor.