What floors me is that, at the end of the day, aren't they in the business of SELLING COMICS...?!? Why would you rather pull something for your "back inventory" (and upset a customer), rather than SELLING PRODUCT...?
If you underprice something, learn from it and be more diligent in the future. And also realize that if you give a customer a deal, they will come back to your store time and again looking for another "score", even if there isn't one to be had. If you pull this , you will likely never see them again.
It varies from state to state, but generally it is illegal to change posted prices...
Example: 12024.2. (a) It is unlawful for any person, at the time of sale of a commodity, to do any of the following:
(1) Charge an amount greater than the price, or to compute an amount greater than a true extension of a price per unit, that is then advertised, posted, marked, displayed, or quoted for that commodity. SOURCE