I'd fight it if the vehicle was strictly parked and not parked "being displayed for sale"... it's all about interpretation, but I believe the ones that are parked specifically to be viewed for sale would be ticketable, but one that is being driven and parked in a normal manner isn't being "displayed for sale". They don't define it in the bylaw, so it is up to the interpretation of the enforcement officer and the judge, but I'm sure there are precedents.
Just my $0.02... from dealing with the red tape a few times