Looking at the shelves, an amulet catches your attention. It is golden, and hangs off a golden chain. The shopkeeper sees what you are looking at and says, "Ah, yes, that is one of my amulets that are on sale for 50$. It's pure gold." "Wow, that cheap!" you exclaim.
"Yes, yes, its been sitting on my shelf for a year now. If its not pure gold, then you can come to me and get a refund." It sounds a bit odd, but you take the amulet. You don't, however, notice the smile on the old man's face as you walk out the door...