I don't know if this is a bug or intended purpose, but some times when object(box, jar, money jar) is with enemies that can make shields, it sometimes shields object which makes it impossible to break.
that is not intentional, it has happened before, sounds like a i missed an instance. You can sell locked items by dragging (like at the forge), its current main function is to not be sold when you do the Sell All option at shops. In the future, forges will get the extra selling options that shops got, but maybe i should also prevent drag-selling of locked items too hmm