During the transaction, yes... it is morally incumbent on both parties to ensure they aren't cheating the other. After the transaction though, the moral calculu

During the transaction, yes... it is morally incumbent on both parties to ensure they aren't cheating the other. After the transaction though, the moral calculus shifts, and I'm going to take into account the time/convenience cost to myself (among other factors) when I decide whether or not to rectify it. (I've found out the hard way that companies can make this surprisingly difficult or time-consuming.)