Experts & Companies on Cashing In Household Items & Collectibles

I’m working on a story that will delve into the various ways to make extra cash with old “junk” lying around your house. I’m looking for experts on this topic and/or companies that buy your old jewelry and give you cash for the gold/silver, companies that make it easy to cash in on old trading cards and comic books, companies that buy back old technology, etc. The sky’s the limit here.