but would like to find out what is the common practice here in Poland.
Well, practically it has to depend on your personal judgement of the buyer. I was always allowed to drive when I was buying second hand cars which I prefer to new ones. The sellers trusted me coz I made a good impression on them, you know, as a reliable driver etc. Besides, .they saw I arrived in my old car.
However, before letting them drive your car, ask for their ID and write down the name and address.