Since you are referencing Football in particular this may be better in the Football forum (
http://www.net54baseball.com/forumdisplay.php?f=8)
As far as Beckett I personally don't think it is a problem to use it as a guideline, but sometimes the market moves before Beckett does also you have to use the guide for all the info. For instance you can't look at VGEX prices for a Mint card and vice versa.
There are other places you can subscribe to that just bases it off of internet sales. A lot of people use
VCP (Vintage Card Prices), but they are not the only one.