In case it sounds like bashing, there is a misunderstanding. The big picture: as soon as I realized the cracks, I have informed the seller. But I wanted to wait for the price of Cruzbike for the seat, in order to ask for a compensation from the seller. Calculating the price however is more complicated, I thought.
I bought the QX100 at EUR 600 including a solid and nearly new front wheel (had been replaced) and pretty new Vittoria tyres. The seller transported it to a friend of mine (20 miles), that's why I wonder whether or not I should ask for a compensation.
May be half the price for a used seat (in case someone is selling a seat in Europe...).
The seller did not know that the seat was broken (because the cracks were covered by the mat). But he had told me, that his son fell to the right hand side (before I decided to buy the QX100).
Usually in Europe something used is sold as seen. Of course the seller should point out known damages espeically if they might endanger the buyer's health.
Still I'm looking for a used (upper) seat in order to test the more upright position, which might be the best for the QX100 geometrie. The Velomo wooden seat, I got by a friend, is too big for my back, plus the max possible seat angle is about 40 degrees.