[biot]

What if a blind person were selling a box of old useless foreign currencies for $2 but when you look through the box you discover there's a couple of $1,000,000 US Treasury bonds (valid, cashable) inside? Do you quickly snap it up, thinking it's a fair trade because you're paying the asking price? Or is there a sense of having taken advantage of the blind person as they couldn't see the value of what they were selling?

I'm not suggesting any sort of obligation towards the junk shop; it's an interesting thought exercise of where one draws the line when there's a knowledge gap between buyer and seller, particularly one involving six orders of magnitude difference in value.