Are all of these NOS tube inventories real? How have we not consumed them already?

Whether it is a dealer or eBay, there is a lot of marketing fluff.  The only way to know the life left in a tube is to have a transconductance and gas test.  That said there are many tube testers in use that are not calibrated and the results are not accurate.  

An old tube that is not correctly tested and sold as good could quickly go bad and take out bits of your amp and preamp.