Legal tender is a term that only refers to the paying of debts. As I understand it, if I owe you $500 and I choose to pay it by giving you a 500 dollar bill, you can't reject it and force me to pay a different way, because I offered you legal tender. Once it loses that status, you CAN legally tell me that I have to pay you back a different way.
That's all it means. Nothing about how much it's worth to a collector, or a bank for that matter.