From time to time I come across articles on this topic. I'm linking one here that is very well-written, informative and helpful.  How to Know If Your Parents’ Stuff Has Value

When faced with the job of cleaning out an estate, warehouse, storage room, attic, basement, etc., don't overlook odd-ball stuff. Don't be too quick to throw it away. For proof of how some "trash" really can become cash, check out this recent article from the New York Times. 
How Discarded Items Become Treasures