I suggest the developers to add a "sell all" button in the shop because I know that a lot of people spend quite a bit of time to drag every item into the shop to sell it.
The game is very fun but it needs a more solid upgrade system. For example the tires would increase the handling of the car or maybe made its appearance change a bit, the same thing with strength and offroad.
 "Monster Legions"){kind=icon}
 "Sands of the Coliseum"){kind=icon}
 "American Racing"){kind=icon}