Sellers who guarantee an item to pass but won't refund authentication fees

[packs]

In almost every case you will get an unauthenticated item cheaper than you would an authenticated item. I will only buy an unauthenticated item if I belive it to be good. And I will take the chance should it fail.

However, if a seller is going to guarantee an item will pass, I don't understand why they wouldn't guarantee the fee. You are offering a guarantee on the authenticity and saying you guarantee it to pass. Why then would a buyer have to be the one going out on a limb on the fee if you're the one who guarantees it will pass?

To be clear, I'm not talking about offering a refund if an item fails. I'm talking about a seller explicitly using the words "guaranteed to pass authentication."