Fiodor,
Its the law to use a notary( its the same in France...perhaps other European countries too?), he has to ensure all taxes, stamp duty and other fees, plus the property is correctly recorded and registered on behalf of the state - so in effect he's a quasi-legal figure, but MUST be involved.
You can use him with an estate agent or also with a private seller - naturally you don't pay a percentage to the private seller, but you do to an estate agent!
PS I stand to be corrected on any of the above points by local Polish posters!!!!.....