It effects them negatively because they let it?
Seriously with over 5 million games sold how was their not going to be saturation in the car market? Sure a few cars will be uber rare like the x1 x2010 prototype and the level 40 paintable one. Beyond that it was just a question of how well you could gold special events for buckets of cash, or just run ovals all day at 3 to 5 minutes a pop.
Some people will have the time to grind xp or cr some people do not have have as much time. If inflation allows some of the more causal people to get cars they love then big deal?
I mean by the time you are level 28 or so you have the races to farm cash or xp, and now we can do it faster than ever since we do not have to reload the entire race and get more cash per race lest until january for the cash bit.
If you want to dupe cars and give out to your friends more power to you if you want to cheat cars and use them to trade well i take a slightly more grim view on that, but all the people doing it will drop the value of the cars period. And seem a ton of people are doing it by the sudden abundance of x1s on the trade forums, but lest the asking price is down to single cars rather than whole fleets of cars.
Far as online goes umm kick the people that are using x1s in your 400hhp race? Beyond that if the car is reasonable and the people driving are not griefers who cares how they got the car exactly if they can actually drive it.