I keep them..my mum keeps them too and she now has a huuuge book collection! Lol. I think she's running out of places to put them all..on the other hand she makes a great library! Do you have any friends you can share books between? Mum often loans her books out, to me, and a few other friends. It doesn't make the real issue go away, but at least it's being read more than once before it gets shelved.
Do you have second hand book stores near you that you could sell them to? They'd only buy them back fairly cheaply though so you're still missing out.