Yeah, there exists a library of data in everyone's game that tells the game at what price items sell at. This is set and is valid everywhere you try to sell items. However, when the Slateport energy guru guy "stupendously goes for it" in low vitamin prices, the data doesn't change even though you bought the items at a lower price. So when he starts-a-selling buy as much as you can seeing as if later you don't need them, you could sell them anywhere and not lose a penny.