My tip number one for UI design: Apple and Microsoft do not pay you to design interfaces, so don't create anything custom unless you're an artist or have hired one.
Some basic guidelines that are easy to follow:
- No custom background colors.
- No custom fonts either
- No wonky canvas-for-buttons
- Use built in controls
- Do not change control paradigms
- Use PushButton, not BevelButton
Yes, there are situations where it's okay to bend these guidelines, but for the most part they're pretty solid.
Specifics for you:
Designing a single-window app can be tricky, and I find some design patterns wander very close to "might as well be MDI" which is what I'm feeling with you first screen. Instead of making the first screen this selection view, make it the "Overview" view and bin this radio-button view selection.
Toolbars should be tools users need frequently, avoid using them as a tab-bar. If you do use a toolbar as a tab bar (like AppWrapper) then all of the buttons should ONLY act as tabs, none of them should be functions (unlike AppWrapper). I'm getting a mixed feeling from your toolbar.
Table column widths should be reasonable for the data they're presenting. It doesn't seem like there's a reason the "Category" column is so wide. Tables should also be easy to understand, perhaps different views for a list of Clients and Dealers.
You have my twitter if you ever have specific questions on things or a "how about this?"
Side note, I don't see how a third party stack view control would fit this interface.