You need to provide the registration card document or karta pojazdu, annual technical check stamps should be on the reg document. You will also need to complete in duplicate a formal sale agreement called a V10, here's one to download
The form includes a declaration that the buyer knows the mechanical state of the car when buying and other things like that. It also shows the price agreed. This document will be inspected later by officials.
You will need to give the new owner the small slip of the insurance document in your name, and then notify the insurance co you have sold the car.
The new owner has to get new car insurance within (I think) 28 days. The new owner should go to their local city vehicle office to register the car in their name using the completed V10, reg document or KP, taking off and presenting the front and rear plates from the car, and then complete a 'Wniosek' form at the city office. A temp reg doc is issued, the permanent one takes a month. The reg fee probably varies but is from 81PLN, plus 2% tax paid when getting the permanent reg doc from either the same office or another office (it varies I think). You get the plates back with the temp reg doc within an hour or so depending on the queue. As seller, you should I think also go to the city office to hand in the sales form and get a receipt for the notification of ownership change (a printed page usually).
You may or may not get an insurance refund for any unused time.
All parties must take suitable ID to the city office - for foreigners they ask for a passport and address registration card.
If a buyer buys a car from a region other than his or her own area of Poland, the registration fee is (I am told) higher, perhaps 100-300 or more.
If buying from abroad, it's even higher than that, perhaps 1000-1500 or more.