If you think that you know how to work a sale, are a pro at salary negotiations, and got a great deal on your latest car purchase — well, maybe you are. Or maybe you’ve been fooled. Maybe you only think you got a great deal. In this segment, we’ll talk with William Poundstone, author of the new book “Priceless,” about the psychology of pricing. From that shirt on sale for $19.99, marked down from $53, to that carton of eggnog ice cream that seems to get a little smaller each year, we’ll talk about the psychological tricks that retailers use to try to get you to pay more than you might want to.