Dime a Dozen

Somebody once told me that his collection of the Hardy Boys books was worth a dime a dozen.  I quickly did the math, and figured out that a couple of bucks for almost $200 books was an extraordinary bargain and saw a great opportunity to make a good profit, and told him that I would definitely buy his set for 10 cents for every 12 books.  He glared at me and walked away.  Some people aren't very good at business.