If the intent of the guy is to do profit with building these midibox, it's illegal.
Thorsten website states that the content of its site is for non-profit personnal use.
Now to sue that guy, it's really impossible because you cannot confirm the guys intent. It's perfectly legal to build a kit and then resale it to the highest offerer because you are not interested to own the product anymore. I've done that with my Fatman, sold it for 80$ (for a negative profit)
However, if the guy would be a company called Behringer and had the intention to mass produce a product based on the design, then there would be an infringement. Remember the square guitar pedals ?
To sum it up, what the guy is doing is shabby, but we cannot accuse him because there`s no definite proof of intention. The only thing we can do is not to buy from it. Hence demand will drop, so the final selling price. We could send a letter to ebay to shut this seller out. But he'll come back under 10 differents names and sell his stuff anyway.
We could discuss about the people who do those cheapo Theremin. They seems to come from all the same original schematics.