Some items can not be sold (e.g limit manuals), this means that they can not be stolen from you and thus not be duped with the help of W-item, but W-item isn't the only way of duping. And items (and materia) are not the only things that can be duped...
I don't understand. How do you know you're getting a battle to get the frame just-on? --- "In peace, vigilance. In war, victory. In death, sacrifice." ~ The Grey Wardens http://www.trueachievements.com/mygames.aspx?gamerid=71900
Random encounters aren't very random, just as with many other random elements in this game they can be predicted.
In the first part of the video I load a save, run into the mansion, and get a battle on the stairs. This means that every time I load that particular save, and run as I do in the video, I will *always* get a battle on those stairs, on that exact step.
Now that I know where the battle will be I then do what I do in the second part of the video, load up the save again, run exactly the same, and then open the menu. I know I timed right simply because there's no battle directly after the menu visit, and from here I just proceed onto the boss knowing I'll have to face it two times.
But how did you know you weren't going to get another battle between the stairs and the safe? And wouldn't opening the menu jar the RNG?
Also in your video comments it says you can't go to the world map so how would that work for Ruby or Emerald? --- "In peace, vigilance. In war, victory. In death, sacrifice." ~ The Grey Wardens http://www.trueachievements.com/mygames.aspx?gamerid=71900
In this particular case I had run the distance all the way up to the safe before I started recording so I already knew the battle on the stairs was the last battle before the boss. Had there been a 2nd battle closer to the safe I then would have opened the menu on that 2nd battle instead.
Yes, opening the menu to remove a battle could make a new, previously not there, battle appear. But it will always change the pattern the same way every time, so if a new battle doesn't appear the first time it wont appear on any subsequent attempts either.
Entering the world map (includes underwater map) will cancel the setup, so unless you find a way to combat Ruby or Emerald without visiting the world map first you won't be duping them with this method.