I sold mine recently – £80 fees to DVLC and the process is straightforward – but not through the ‘Sell you Plate’ type websites. It was registered with one of those and advertised art £3,200 of which I would get £2,500 but I heard nothing from them. Strangely they already had it listed on their website even though neither me, nor the previous owner had spoken with them.
I actually sold it to a guy who started talking to me while I was stuck at the side of the road with a puncture on Christmas Eve. I gave him my number and told him to call in January if he was still interested, not really expecting to hear from him again. 10 days and £2,000 later I had sold the plate.