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What is cheap way to go from Krakov to Berlin, but I can purchase ticket before train departere only (because arrive from Lemberg)

In case of any trips from Poland to Berlin you need to take advantage of one general tip - use Polish domestic tariff for maximum distance possible (instead of more expensive TCV tariff).
While planning overnight (or daytime) trips from Warsaw/Krakow to Berlin get polish domestic ticket till last Polish station before German border - Kostrzyn and commuter train from Kostrzyn to Berlin (11.8 euro).
In your case it is 62 PLN (15 euro) for Krakow-Berlin part. Train from Krakow arrives to Kostrzyn at 6:31 am, afterward wait till 7:05am and board commuter NEB train from Kostrzyn to Berlin. You will be in Berlin at 8:28am
As a result you will pay only app.27 euro for the whole journey from Krakow to Berlin, instead of 50+ euro for the full ticket Krakow-Berlin. And what is more important timing of your trip will be almost the same.
Same tip applies for travels from Northern part of Poland, but there you can as well break your ticket via Szeczin too.
 

Thank you TUT, but where I buy Kostrzyn-Berlin ticket?

You purchase it in Kostrzyn or from the conductor on the train.
You can buy it even in Warsaw, but then it will be issued with a help of regular international tariff (TCV) - means more expensive.