While I don't collect sportscards I DO dabble in comics and have found that Ebay is the best indicator of what things are ACTUALLY selling for. (Used to be a pretty hardcore seller up until a few years ago before Ebay/Paypal fees and Canada Post greed put me out of business.
These days Guides are wishful thinking books because the back issue comic book market has been depressed for years and only the premium, hard to find stuff is getting anywhere near guide value. Everything else you're pretty lucky to get 30-50% of guide.
I do have a few friends with feet in both markets and say conditions are pretty similar.