I found this, and I think it might come in very useful for anyone looking to buy a property in Poland.
https://ekw.ms.gov.pl/eukw/menu.doYou need the so called "księga wieczysta" (property number) to access the data, but when you do, you can find out a wealth of information about the property. It contains details of previous owners, current owners, mortgage on the property, everything.