This is simply arbitrage. The book sellers don't take the time to do the research to price their books accordingly or offer them for sale online which creates a profit margin from the price difference between the two markets. He's not marking them up to some unreasonable level and controlling the book supply - he's correcting the sale price to reflect market value.

The book sellers are obviously missing potential revenue here. Eventually they might wise up and start doing this themselves, or maybe a startup will come along to help them with this, cutting guys like this out.