If you live in a city of any size, thousands of people see your license plate every day. The average person cannot look up your personal information using your license plate number. Drivers information is restricted by privacy laws.
Two ridiculous answers so far. Who would advertise a car for sale without telling where it can be seen or how to contact the owner? Anyone who wanted to could call, ask where the car is located, drive by, and get the tag number. I have to ask where you live and which selling sites you're looking at? I don't remember ever seeing ads with the tag numbers covered and I have looked at a lot of cars.
1) Information about the car and previous owners can be found which could lead to the car being damaged / stolen or vandalised.
2) Plate cloning, people will check the car model, make and color and can buy a personalised plate using the car registration plate number found on "your car" and put it on a similair car in order to commit crimes, such as robbery and most common is filling up gas at the petrol station and driving of. The cameras would be played back and the car color, model and make will corespond to a differnt car such as yours and you'l get a fine.
Like if you had a blue bmw x5 with a legitamate registration plate, and another person has the same car obviously differnt number plate, he could clone your reg plate, by using "illegal registration plate" websites to create a reg plate with your reg number and letter which isnt authorised by the DSA/DVLA/DMV.