You gave him the chance to inspect it a second time. That was the moral thing to do. I don't see the point in chasing him down to tell him he was wrong. He got it wrong twice. Not your fault. If he was comfortable with the deal after giving it more than one close look, then it's his loss.
Since you already own the book, I don't see how he would be able to charge you extra for it. That being said, if it relieves your consciousness to tell him about it, then do so! If I were the dealer in question, I would simply have to learn to live with the oversight, and I'd let you go on your merry way.
It's both the dealer and the customer's responsibility to be as thorough as possible. It's easy to miss things, and it happens all the time. I don't see any willful deception on your part, or the part of the dealer.
Had I been in your situation, however, I probably would have mentioned something about it after checking Mycomicshop.