One time, me and my friend came across a book that was only sold through a small publisher in Omaha. The book sold for ~$20 and, since we couldn't find it on Am

One time, me and my friend came across a book that was only sold through a small publisher in Omaha. The book sold for ~$20 and, since we couldn't find it on Amazon, we resold it for $300 a pop. Every time we received an order on Amazon, we would just take that buyer's information and buy it on the small publisher's website—we essentially took on zero risk and worked a few minutes every time we got an order. It was a great example of arbitrage. And just like arbitrage, competitors also saw this exploit and the market inefficiency disappeared. Margins were competed away. Even though it was a temporary window of opportunity, we found an article saying that some people do this kind of work full time (finding obscure books and reselling at a huge markup)!