Dazzle there are full instructions on the websites - you just type in the barcode on your CDs or DVDs, or the ISBN number of your book, and they give you an instant quote.
I've found both these sites better than Music Magpie, mainly because they suit the clutter I have!
I usually have windows of both Fatbrain and WeBuyBooks open at the same time on my screen as the prices they offer you can vary quite a bit. One CD I had got offered about 23p from one site, but £2.50 on the other, so although it's a pain, it's worth doing this.
If you accept their offers they email you a packing label, with their address on it to stick on your cardboard box of goods. Then just take your parcel to your nearest post office or Collect+ outlet (eg Value convenience stores - you can search for your nearest one). You don't pay any postage, and you'll get a receipt when you drop off your parcel, so you can check the progress of your parcel. We found empty wine boxes from our local supermarket were a good size for getting CDs and DVDs and a few books in.
I have to say it's a bit hit and miss which books/CDs/DVDs they take, especially the books. Often you'll get a message saying 'sorry, we don't need this title' - chick lit and crime mostly, they must be swamped with them. But we've sold a few classics, and coffeetable (glossy picture) books did well, as well as non-fiction. Haven't tried kids books as we've used nearly new sales for those.
I've found both websites send a cheque within a week of me dropping off my parcel. Haven't tried BACS payments straight into my account. They send you emails to confirm when your parcel's been received and when they're sending your payment.
If you check out WeBuyBooks on Twitter they often have voucher codes that anyone can use. I used one the last time I did an order and got an extra 10% on top of my offer - an extra £5, not much but equivalent to a lot of CDs at 30p a time!
We had a good clearout last summer and got about £120 altogether in three or four boxes - a nice addition to our holiday spending money.
Hope this helps, and good luck!