If the car was traded in at a franchised dealer, then off loaded to a smaller used car dealer, then re advertised, they don't like showing how much more they are advertising it for compared to how little the person who traded the car was given. So they blur the plates out, so it lessons the chance of the previous owner stumbling upon their car online/in the classifieds, if it is advertised by a totally different car yard ( other than the one they originally did the trade in with ), it could be just any other car, not theirs... Makes the HK customer feel better. It also removes questions like: ' how come my old car has 100,000km less mileage on your advert than when I traded it in with you ? ' Winding clocks back is a prolific habit out here...
Example: Customer has a 10 year old BMW 5 series to trade in on a new Mercedes Benz E350 at Zung Fu, they trade it in for 9/10ths of stuff all, the dealer rings around the used car trade and off loads the old Beemer to the highest bidder. The Beemer ends up in the hands of Vins/Vincent Motors/Expat Motors, European motors lets say and the plates are smudged out in any new used adverts with a MUCH higher price than what the original owner was given at the time of his/her purchase.
It is a bit of a childish practice, but that's how it works in the HKG..