I recently sold old books, DVDs and CDs to Music Magpie, WeBuyBooks, Ziffit and Momox.
I have used Music Magpie in the past and have been happy with their service, although the payment used to be slow.
Having discovered the other companies through the MSE webpage I entered the ISBN codes in all of them and simply sold to the one making the best offer.
MusicMagpie and WeBuyBooks offer the easiest (free) postal service with home collection. Ziffit and Momox offer a drop-off service only.
MusicMagpie confirmed receipt of the parcel exactly a week after it was collected and paid on the same day by bank transfer, accepting all the items. The payment was on my account the following day. Excellent service.

WeBuyBooks and Ziffit confirmed receipt 6 days after posting. Ziffit accepted all the items on the same day, providing a Paypal payment. WBB took 3 working days to check the items (accepting them all) and provided payment by bank transfer immediately after. Both payments arrived on the same day.
No news from Momox two weeks after posting. Since they are in Germany, however, they will naturally take longer. I e-mailed them and they said that it can take up to three weeks for the parcel to reach them. I will update this in due course.