I did 3 Atari 800XLs, and one Vic-20. The 800XLs were not really noticably yellowed, until AFTER the cleaning and they were snow-white again. You forget how things looked 25 years ago unless they're really bad.
However, my recently-acquired Vic-20 was what I might say "not so good." In the tiny Ebay picture, it was difficult to see so well. I definitely did NOT want my first Vic-20 to be a yellowed turd, so I thought I was getting a nice one. It was the newer model with C64-style power, complete with box, guaranteed (or at least assured) to work, and reasonably priced! What's not to like??
So it gets here.....box isn't like new, but that's ok, it's not bad. It's what's inside that counts. Many times, when I've ordered old computers complete with box, there's a good chance that (1) they've spent a good deal of their life in the box, or (2) they were owned by someone anal enough to keep the box, which means they're inclined to take care of things!
Well, surprise!! It was filthy and yellowed!!! I had been avoiding yellowed computers for a long time, even after I heard about the "Retr0brite" (peroxide) stuff because I didn't know anything about it. (I even bought a non-yellowed case bottom for my Super Nintendo from Ebay because I didn't know about the peroxide stuff). I was horrified! Not only that, it was missing the foam inserts for the box!! AAARGH!!
[anybody have a couple foam inserts and power supply box for sale??]
It worked, and was pretty much how the seller described it (although not what I was expecting) so I left positive feedback. Nothing left to do now but try and de-yellow it!
Well, I'll be damned if the process doesn't work!!!! I got what I wanted - a SNOW WHITE Vic-20!!! I am so thrilled! I don't have space to keep all the computers (C64, Ataris, Vic, etc) all hooked up, and I'm waiting on a Mega-Cart, so I put it back in its box - sans foam inserts, of course. But I keep taking it out of the box to admire!! It looks so great!!! I encourage everyone here who's been thinking about it to DO IT because I'm as amateur as it gets, and I did it!!
I sent in these pics to the Retr0brite wiki, and I'll post them here!
Here is my Vic-20 before:

Here is after:

I can post pics of all the stuff I used - which is readily available. I used thickened "developer" for bleaching hair, rather than what the Wiki says.
You should read THE RETR0BRITE WIKI if you are even remotely curious about this.
Be sure to click on "the gallery" on the left - and scroll towards the bottom to see my Vic! However, there are far more impressive before and after pics than mine - including an awful-looking Vic and many Commodore computers.
Note: I know this info may be old to some and is available from many sources, but I just want to encourage people, as it was other people's similar testimony, pictures, and discussion that got me to try, and I'm glad!!
My Vic is now Mega-Cart worthy. [fingers crossed in anticipation of Mega-Cart]