Another version of this method is drop shipping books from Amazon to your eBay buyer. This actually happened to me and I figured out the method.
1) Get an Amazon Prime account (free for students if you have or can get a .edu email address)
2) List books on eBay that you can drop ship from Amazon. Mark up the price 0.50c to a dollar. Use an eBay calculator to figure out how much profit you can make.
3) Set your eBay shipping price at 4.99.
4) Complete orders using the free shipping provided by Amazon.
5) Collect your $$$
Extra tip: what caught my eye on ebay was the seller said free upgraded 2 day shipping. 2 day shipping compared to media mail (5+ days) was an easy choice.