Try and sell something to a dealer. They can't grab the Beckett fast enough to see what your cards list at.
Beckett is dead on on some items and way off on others. Check ebay completed listings as an example to see what people are really paying for any said card. I personally don't think the guide is a waste if it is used properly and all information taken with a grain of salt. That all said, anyone can ask whatever they want and you can pay whatever you want, even if it's 4x book and you really want it.
Hope that helps a little!