You won't get more than €2 or so per book from any second-hand shop. Possibly much less. You'd potentially make more selling on ebay but the time you'd have to invest into it might well render any profit negligible. I've sold books online before and it takes more time than it's worth, in my experience, between putting the listings up, dealing with questions, researching prices and postal rates and then having to package and post the damn things!
Best of luck with it anyway.