I'm a sucker for romantic fantasy novels and buy em by the shed load on Amazon, (my very fave market place). I rarely buy my books from Amazon itself cos I'm too tight, instead I use their sellers market place and get them for pennies. Once I've read them I sell them on using my own book store on Amazon which I created a couple of years ago (KDS Books). Paranormal romance is a hot seller so they go quickly.
its a win win situation because I get my books for practically nothing, postage is more than covered by Amazon postage fees and sometimes I've even made a small profit.
Its really simple to open an amazon sellers account and doesn't cost you anything unless you go pro, also they pay you straight into your bank account every two weeks which is nice. First time I ever did this I had a load of books to sell and sold 20 overnight (and that was as a new seller).
You don't have to stick to books though, if you have anything else you want to get rid of just sell that, use your imagination. Books are good though and if you dont read prolifically like me, then get yourself down to your local car boot sale/flea market and snap em up for pennies.
To be honest I prefer Amazon over Ebay any time.