Is there a look back period? What stops me from selling my business to my buddy the day I leave and then buying it back the day after?

The idea is that leaving the country is taxed as if you sold your shares.

So selling them presumably doesn't help.

Your taxes should work out the same (or worse). Exit tax is akin to you selling all your properties at the point of exit.

Yup, this is possible. It would have to be at some fair market value, and you'd (obviously) have to tax that in Germany. And depending on how much you trust your buddy, you might or might not have to draft up some complicated legal framework that you indeed have the right to buy back your company at some stage :)

Nothing. But you will have to pay taxes on the money you get from your buddy. If he is paying too little, you may get into additional trouble.