Here's an invention that car lovers (especially teens) will buy. If you can figure out how to build counter-rotating hubcaps, you'll sell a ton of them. In other words, when your car is going forward, the hubcaps spin backward. It might seem silly to adults, but this is exactly the sort of thing 17- and 18-year old car owners like.
Alternatively, you could invent a hubcap that remains completely still. That would be pretty easy to make. All you'd need to do is have a hubcap that spins freely on a bearing and then weight it on one end so that the heaviest part always remains at the bottom. Get this to work, and you can probably sell 100,000 of them for $39.95 with no problem.