For what it's worth, I've dealt with tonyetrade before and did not find him to be overpriced.
It is true that collectors will often happily pay a premium for the specific example that appeals to their eye. Whether one is a stickler for centering, registration, corners, etc., we all know that not all cards with the same label command the same price.
The PSA 4 pictured is a very clean image of Mantle. That said, you can definitely improve on that centering, if centering is your thing. This is only a 3 but the eye appeal is pretty strong...
