try and find some really really cheap ones, Big uni's tend to use the same books every year etc. so not only is it more demand, its also more supply to flip.
Most students would sell books for quite cheap as they don't need them, then you can just sell them on.
I know what you mean about getting money tied up though, but if you make your sale first, then buy (if you have a reliable supplier/way of getting books) its easy-ish to avoid.
But im just trying to give examples to people to think outside the box. I'm sure there are other great markets which haven't been tapped yet!