Can you honestly say that building a Midibox Sid like Wilba mkII is any easier than anything what I have failed before?
Well, did you have the designer and several other knowledgable experts and a few thousand experienced people helping you last time? Extensive documentation? Realtime chat support?
In case you can’t tell, that’s a “YES!” 
I guess it would be like two or the boards looking ok, but not working and I will have no clue what so ever which components are dead and which are not.
That’s why we have troubleshooting guides and applications, if it comes to that. Most of the time, it doesn’t, because of the hard work of people like TK and SmashTV and Wilba. That’s the same hard work that it’s frowned upon to profit from.
It will end up on the recycle center.
Unlikely. I think that’s only happened to like two people, and only then because they refused to take the advice they were given (ie, they were told “do this, to fix it”, and they refused). Occasionally something gets fried, but it’s rarely a drama to find the problem and fix it.
I see the point that TK would take it as an insult if a company picks up the designs and makes profit. But if somebody want to sell one of their creations(not to start a business) to make another, just cause they are bored with it or whatever… what is the harm?
What is the harm? Well, that would be a question only TK could answer, and so that person should Request his permission to make the Sale, in an appropriate forum, like say, Sale Requests (You are HERE) 
You said you don’t see any for sale here, and that’s because there aren’t any for sale.. with this one unpleasant exception. Really, the only reason to not ask TK’s permission, would be if you knew you weren’t on the level, and would be denied. Usually, it’s encouraged to sell the box here. If this person only wants to sell the box at cost-price, and is happy to sell it on the forum, historically that is the kind of thing TK has given his approval to do. You can find a few examples on the forum here - although there aren’t many, because people tend to hold on to these things if they’ve invested the effort in creating one 