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it's very hard to imagine But the Marketcap was 820 Billion in December and he sold off 400 million it just dosen't make any logical financial explaination, were talking over 400 billion drop on fear of someone selling 1 billion ?

The marketcap is just a representation and interpolation of the current trade volume.
So it was only theoretically at 820 billion. This number might be way lower.
Tether is also contributing to the fact and you don't really know how much it is actually backed by real money. We might be looking at 1:10 , some people believe its 1:50 or even worse. If you consider this fact it doesn't look too suprising anymore that the market is in depression after 400 million USD worth of crypto were sold.