Law / Buying an unregistered car in Poland? [9]

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Driving an Unregistered car in Poland, from Poland to an Eu member country, on a trial plate

To make this process,what i need ? I mean about the plate,the trial one; how does it work .
At the conessionary they told me that i legal to buy a car,not register it in Poland and just drive it to another EU contry and then make the registration in that country.

Can you give me all the information that i need ?
Thanks on advance