Ive been selling books on amazon for a few years now. Books are easy to come by, and easy to store. This video explains everything you need to know to start selling books and making profit. Start with books you have laying around the house, then you can branch out into garage sales, or find large batches of books from Craigslist or other websites.
The cheapest place to buy the bubble mailers (to ship the books) is on Amazon:
This is cheap scale that you can use to get the easier shipping on Amazon:
Let me know if those links stops working.