Just depends what kidna business man you are. If you're running a shop selling magic, then it is your job to sell magic cards. Of course the details of that job include sorting, grading, and such, but pricing is part of that job. If you aren't keeping up on those prices thats your own fault for not doing your job.The stores around here are good about it. I stay on top of prices, when something goes up I'll check if they've changed the price or not. If they haven't I'll scoop it up if the price difference makes it worth while. I've been asked if the price has gone up on cards I was purchasing and once the store owner sees the price difference I'm still allowed to purchase then at the marked price and they correct their error on pricing.
Its win-win. My buddy owns a store and I grabbed up an $8 card for $4. I let him know about the price change and he let me know that he didn't care. He already had his mark up on the card and made money on it, so there was no issues on his end. If he was happy yesterday selling at that price, he'd be just as happy the next day, but would also price that card correctly in the future. He made money, I made money, it all worked out in the end. Not being greedy caring if others can spin a profit sure helps a lot.