Hello,
I'm writing from Poland (sorry, I don't know German) and I need some help! I
would like to know if it's possible to sell mushrooms collected in Poland
on the German market.

I cannot answer all your questions. In Germany it is strictly forbidden to collect mushrooms and sell them here. So it seems to be essential that you can prove with a suitable documentary that your mushrooms are from Polish origin. Prices I do not know exactly. 100 g for 2,50 Euro is what I remember for Craterellus. In the big shops there are Boletus sspec. in addition.

Can you, please, tell me what are the regulations if any? Are there certain
purchase points I can bring the mushrooms to? Do I need any paper work?
For what kind of mushrooms is there demand and most importantly what are
the prices?