Anyone used ziffit to sell books? Are they any good?

In mymission to get shut of a load of crap I have done all my CD's via muscmagpie now looking to get rid of all my books and have come across the ziffit site - they have an app so you can scan your books in to get prices etc - was just going to give them all to a charity shop or similar but reckon I will give these a go.

Probably have around 500 books which are pretty much useless to me now as I use a kindle to read on