From "you suck at craigslist:" 2+2 = 5. People selling things on craigslist who want $x for one of some item and more than $2x for two.
I'm pretty sure I once saw somebody selling T-shirts on the boardwalk in Atlantic City for "$3, 3 for $10." Or it might have been "$4, 2 for $10" or "$2, 4 for $10". In any case, it didn't make sense.
I'm sure there are cases where this sort of pricing structure actually makes sense, but I can't think of anybody. (I also wouldn't be surprised to learn that economists have a name for it.) Anyone want to try?