I've tried this on a spare slightly yellowed 8D8 figure that I have. The body of the figure is as white as the day he was made, but the limbs and head had yellowed.
I put him in 6%, 20 vol, Hydrogen Peroxide and within a few hours I could see the difference. It's said that the sun can play a big part in reversing the yellowing effect, but as it was late evening (and no sun), it's just been the peroxide at work.
It's a great trick to get figures back to their former glory, but as it's already been mentioned, it won't be long before the ebay crooks catch on. I can see the quotes now in EVERY crooked sellers listing....
I can GUARANTEE that these figures HAVE NOT been dipped in Hydrogen Peroxide or Retrobright.