When you send in your game, you declare a value. That should be the value they declare in case of need for insurance. That's on YOU, and rightly so. If you send in a game "worth" 20k and declare it as 2k,then it burns in a fire, you're getting 2k from insurance, and that's a YOU thing.
It's bullshit that they'd try to tell anyone the value of a second hand market item. They should take your game, give it a grade per their service, and put it in some plastic. It's not a valuation service, and definitely SHOULDN'T BE.