on the cars -- horses were bought and sold. trains. boats. you name it. buying a product that is transformed by technology is not the same as trying to take a service and transform it into a product through technology. you can augment the service (large margins) or automate the service (thin margins), but you can't turn it into a product.
on the sheet music -- precisely! that's the exact analogy. note the incredible consumer demand for sheet music today! it's not exactly a booming market outside of the niche training/instruction arena.
edit: btw, that last bit about the incredible demand today for sheet music was sarcasm. just in case it didn't come across....