This would be a perfect way to determine the value of each and every coin if the prices listed in these guides were accurate to begin with and if coin values never went up or down. In the real world, of course, prices are in a state of constant flux and many of the values shown in guide books and listings in hobby periodicals are already inaccurate - sometimes by wide margins - before the publications even go to press.
Highly-respected coin dealers are honest and try to give their customers fair value by using the most up-to-date price guides available to them, plus the best information available to them from other sources, including auctions, fellow dealers and multiple dealer electronic trading networks. They are best suited to do this for the customer in the areas they specialize or make markets in. But even the best intentions can be undermined by fast-moving market developments that only the most connected market making dealer can keep up with. Meanwhile, dealers with the worst intentions view this situation not as a concern, but rather as a golden opportunity to buy very low or sell very high.