Overall, looks pretty good.
The one thing I would work on is that you're kind of abusing gradients for lighting/shading. You should try using more hard shadows, and staying more consistent with your lighting direction (some of your gradients and shadows don't make sense, like the body of the cart being lighter on the bottom).
I'll try to include some examples later (I'm at work right now) to show you what I mean.
Also, I can't quite tell what's supposed to be interactive- maybe use slightly thicker lines around interactive objects?