I am of the thinking that the concept of "Karma" exists. It is included in every "real" religion" in the world. "What goes around, Comes around"/ "Good brings good, Bad brings bad" type of thinking.
As well as the "Golden Rule", being, "Do onto others as you would have done to you."..
So, I try to be as honest in my dealings with people, regardless of the situation. That is "me".
I think, had I been in that situation, that after telling them or showing them the ad with the different price & they offered to sell at the lower marked price, I would have bought the amount I originally went in to buy & not "all I could get", so I would be leaving some for others, or allowing them to correct the mistake they made.
If the mistake had been the other way around with the advertisement marked at $47.00 & the cost in the store at $74.00 then situation is reversed & then that "turnaround" would have to be considered by the store & the buyer both as well.
The "rule of life" I use & mentioned in the 1st paragraph of this post would still apply for "me" for both situations. For others... "YMMV" I reckon..
If you are comfortable with your dealings, then good for you. I hope they shoot great for ya!