I sold two lots to MM - the first one was fine, but after the second one, I vowed never to use them again. They offered me a price based on the actual barcode of a CD single, then declined to pay as 'they don't accept CD singles'. Also, there were two other CDs that they claimed I hadn't included. I had: I checked it about six times. Absolutely nothing I could do whatsoever. The total of the 'disputed' items was only about 20p, but for me, all trust as to their integrity had completely gone.
I also sent some things to WebuyDVDs, who offered a little more than the others, but they took absolutely ages to pay me; I had to chase them several times, before they eventually paid me, but still insisted on (taking even more time and) sending me a cheque. Wouldn't use them again.
By contrast, I sent some items to Webuybooks (who will also buy some other items, such as DVDs) and I found them to be honest, prompt and efficient throughout. I'd definitely use them again.
Never used Ziffit or the other one - I forget the name, is it Zapper or something? - so no idea about them.
I would advise to only send them items that they offer up to about 50p for, though, and consider selling the others on eBay or similar. They build in the cost of running the website, admin, staff, paying for the postage and profit, of course - nothing at all wrong with that; but their economies of scale mean it's worthwhile for them to do it but not for you to bother.
However, if they're offering you significantly over £1 for an item, it's a reflection of its rarity rather than their random generosity(!), and it might mean that you could get a tenner or more for it yourself. One of them (I forget which) offered me £1.50 for a CD and £2.50 (IIRC) for a book and there were a very small number of them available on eBay, going rate seemed to be £30+ each. Now I just need to get around to listing them, along with loads of other old junk