long story short is deals are everywhere, just takes time learning how to spot them, and get idea of what sells, and is shit.
Used books is buy far the easiest way to get into amazon. Book sales at the library normally happen once a month. They have a bag sale on the last day, where you fill up the bag for $5 bucks, meaning each book is about 20cents, and if you choose right you can find least 10 bags worth of books you can sell for 8 to 15 dollars.