Personally background cars aren't irritating me (especially in old movies) even if they are 'small', but quite visible and on the good quality pictures, however I understand the problem.
So maybe every time when we are adding not well visible background cars we should give info why we are doing this, what is the reason, "I've listed this car because there are only 2 on the site" etc. Probably everyone will think twice when he will have to justify his choice.
The more drastic method (if it's technically possible) is creating the limits for background cars in the film, e.g... 10 for movie, 20 for TV-series or something like that.