A car dealer (friend) told me that for an ad posted for vehicle sale, most of the time (9 out of 10), its a dealer trying to sell a faulty (with reversed mileage, accidental, or mechanically poor) vehicle. A private seller will never ever hide the license plates (he is not selling a stolen car or whatever).
Beware buyers! If someone hides the number plate, he is actually hiding a negative point about the vehicle, which may directly effect the sale or sale price. That's it.