I don't know if this would be practical, but a nice curtesy would be for the authorising agency to verify that there aren't any outstanding parking tickets agai

I don't know if this would be practical, but a nice curtesy would be for the authorising agency to verify that there aren't any outstanding parking tickets against a newly registered plate, and to at least advise the purchaser, or refuse to issue the plate. I can imagine human ingenuity is such that there are a huge number of possible 'bad', placeholder number plates used - in addition to the 'NV', 'NULL' and 'NO PLATE' listed in comments, imagine we could have 'NONE', 'NA' (i.e. N/A), 'BROKEN', 'ABSENT', etc.