can you explain more about how the arbitrage works ? I am confused how it is possible. for instance, I buy a lot of used books from amazon.com (well, third party sellers). when i one-buy click, the purchase goes through immediately, and the sellar already has the book listed, so i am confused how it would even be done.
This is very easy and actually quite common between Amazon and Ebay. You would list the title as Merchant Fulfilled on Amazon and when someone orders it you would place the order yourself on Half.com using their shipping address and pocketing the difference.
Doing it manually is one way but not scalable. You can aggregate data either using available APIs or by scraping websites. You can list books on Amazon that are available on Half.com and give you a decent profit margin. It doesn't matter if books are already listed on Amazon. Just make sure your offer price is a little bit cheaper than the cheapest available copy.
There is so much potential out there.