Every market is like that -- you really have to get in and look around, try a few things and see what works. The classic example in selling online is the guy who goes broke trying to sell iPods, vs. the guy who makes a good living selling iPod screen covers. The "visible demand" market is usually the one with the most competing sellers and tightest supply pool. With books, that would usually mean your high-demand bestsellers. But look at the movie market and other forms of media. Has some timeless classic recently been made into a movie or television show? Get some prices on printing up some from Penguin, add some artwork sourced off of DeviantArt or Freelancer.com*, sell them for next-to-zero markup, then upsell your customers on bookmarks, reading glasses, tote bags, Kindle gift certificates, copies of the older movie version of the same book, whatever you can source really that ties into the main product.
*added note to concede that yes, it's slightly more complicated than that, but if any work is no longer covered by copyright, the only thing preventing you from just publishing your own copy is your own hesitation. Self-publishing a classic book is a fairly straightforward process, and you can have a finished product available with very little out of pocket, thanks to an abundance of "vanity publishers" who have sprung up in the last couple of decades. Margins on the products, unless you are buying them in quantity, will tend to be vanishingly small to just-barely-covered-the-shipping -- but what those products do, is get you a list of people whom you know for a fact buy things online, like a certain kind of book, etc. and the upsell/repeat business is what you're really targeting.