It's called marbete and it serves as proof that the car has passed a (good for nothing) technical inspection and that the obligatory liability insurance has been paid for a year.
Without marbete you can't take the car on the road - fines for driving a car without a valid marbete are high!
You can easily check if the papers are OK by asking for the license which should be valid (it should be renewed every year and after paying the yearly fee + fines a new license and a new marbete is issued)
Also the official owner is mentioned on the license.
Now, before you give the previous owner a penny, go with them to a colecturia or the DTOP where the car can officially be transferred in your name. There they check if any unpaid loans with the car as collateral exist and if there are any fines on the car. The seller's ID and your ID (driver's license is best) are checked.
The 'old' owner has to pay fines and if the transfer has gone through you know that the car is officially in your name, you know it's all legal, there are no more fines to pay and you will also receive a title.
Next you can pay the seller and get the car in your possession.
Never ever buy a car and pay the seller without having it transferred in your name!