I have some confessions to make too. I did what this guy did but in a much more automated, scalable fashion. I did online book marketplace arbitrage.

Basically, I wrote software to automatically find books for sale on one used book site on the web and listed the same book for sale (with a markup) on another site. If my listing was purchased, my software would automatically purchase the book from the original seller and have it shipped directly to my buyer.

In the second half of 2007 I did over $200K in sales and had 10s of thousands of books listed for sale at a time. If anyone has any interest, I might write a blog post about it.