If it were easy, we'd all be doing it
I'm no expert of graded cards, but from what I gather the graded cards that book abnormally high are due to rarity/scarcity. Take the 1971 Topps Bert Blyleven RC. Ungraded, in NRMT shape, it books for X. The same card graded at an 8 books 10x higher than the ungraded one.
http://www.beckett.com/baseball/1971/top...rc-3730653
Another thing is that even if you had a good shot at, say, 50 Xander Bogart or Manny Machado Bowman Chromes, only 3-4 would probably be gradable. Of those 4, maybe 1 or 2 would come in at a 9.5. In the end, it's all a gamble. 4 or 5 months ago I picked up a lot of 75 Avisail Garcia chrome cards for 10 cents on the dollar, which was a good deal. Granted, it's not a hugely valuable card (either then or now), but if I have a 9.5 or 10 in the bunch I'd probably do pretty well.
If you want to make a little money is baseball cards you're probably better off scouring the bay for misspelled listings and other chance opportunities. -For example, I recently was bidding on a Verlander (base) card I needed for my PC. The listing said it was a lot of 2 and described the second card as having a "greenish" tint. I didn't pay it much attention (and the scan did not make it apparent that the card was actually a green refractor), but I bid about $5-6 and ended up with a $20 base card and an $80 parallel. Personally, I wouldn't feel comfortable straight-up taking advantage of someone's mistake, but there are often opportunities to nab a card at a great price. I follow the listings and will see a card that regularly sells for, say $25, selling for $12 for no apparent reason.
In any event, rest assured that there are plenty of pros out there trying to do the same thing. I'd suggest just have fun with whatever you're doing, make a gamble every now and again, and stay within your means.
Always looking for Verlander, Cabrera, Maybin, Mike Stanton (marlins), and Avisail Garcia.
*TRYING TO COMPLETE MY VERLANDER ROOKIE COLLECTION. 44/47. ONLY 3 TO GO!*