I've had a look through the reviews and I think Ziffit are getting a bit of a bad press for not being who they say they are. However... they state clearly that they can't send books back if they are rejected, I knew this so I only send books that I would otherwise have given away. If you have a book you would rather sell than get rid of for almost nothing, sell it on Ebay or something. You won't make a fortune for your old books but it is a pain-free way of decluttering.
One thing I did take issue with is the courier company they use. Both myself and my uncle have seen the courier drive by without stopping to collect! If they used a more credible courier company then I'd have gone for 5*.