I just took a look at Operation Bullpen; interesting stuff! I see your point and it's interesting how they fooled some serious collectors, etc. I also would fail to spot a counterfeit a $1 bill for the same reasons, but there probably aren't even that many out there. Bills have to be redesigned constantly to stay a step ahead of the forgers, and every bill has been redesigned since the 60's- except the $1 bill. Therefore, the government sees that forgers aren't faking the $1 bill and haven't exhausted any resources to stay ahead of the counterfeiters, because there aren't many. You might very-well see low-end fakes at swap meets, etc, but not on ebay and here's why. The listing fees are too high to sell $3 fakes for little profit. Paypal takes a cut and listing fees are per auction, and the card might take 5 times to sell if there's no demand. It's not worth those listing fees and paypal cuts to sell a low-end fake on ebay. However, I wouldn't trust a $1 auto at a swap meet or some other forum where it costs nothing more to have more cards for sale and there is nothing lost if the card doesn't sell. At swap meets, I would say that any auto is fair-game to be forged, but much less so on ebay.
So exactly how much "profit" do you have to make, to make it 'worth it"? To each their own, right? And you're thinking small- most of the guys who do forgeries don't do just one item, they do HUNDREDs of them in a day, and flip them all over the country on unsuspecting dealers, collectors & investors. Operation Bullpen was mentioned, look that up and see exactly how the Marino brothers did it. One day they would forge Mantle sigs on hundreds upon hundreds of items, then the next day they'd do DiMaggio sigs on hundreds & hundreds of items, then the next day another sig, etc. And again I point out those are items which fall under intense scrutiny from buyers. If you could do an item that falls under less of a watchful eye, make less profit but operate for a longer, uninterrupted period of time, then the same profit would be there over time...
As for your counterfeit bills comparison, I'm not sure it applies. Could you spot a counterfeit $1 bill? I couldn't, I barely look at $1 bills. Same theory for common player sigs- look at the example here- your eyes go to the rarest, toughest, boldest sigs and barely glance the common ones. And if you did look closer, would you know the difference between a good Alvin Davis sig and a bad one? I wouldn't and couldn't use that as a reason to doubt the ball..