Just like real estate has three major criteria (location, location, location), so does book buying: condition, condition, condition.

Once the condition meets all your requirements (see our glossary for some definitions), then you need to consider the content. The question I ask myself is "Do I want to read this book?" and if I say yes, I then ask myself if I want to read it now or someday. If I tell myself I want to read it later, I price it so it is so expensive no one will even think of buying it from me, especially since there are a gazillion penny copies already available. That way when it sells (and over-pricing it just about guarantees someone will buy it before I read it), I'll make a handsome profit and then go buy one of those penny copies.

Then there's the genre. Do you specialize in anything? As a rule, nonfiction sells better than fiction and if you specialize in a particular area (military, social sciences, children's books, nautical or aviation, etc.), buy everything you find on that subject.

Get rid of the "double your money" philosophy. You will have to carry your books in inventory for a long time before they sell and shelf life is an indirect cost. The lower you can keep your purchasing costs, the easier it will be to sell for a price that gives you a good return on your investment.

But more importantly than anything else, it is mandatory that you have a good reference library of books about books. And you can never have too many.