Alex, awesome photos!
I dabble a bit in photography as well and the best advice I can give is to concentrate on composition and lighting.
Composition
Is there something distracting in the background?
Did you cut off the rear quarter of the car?
Are there service mats or dust rags visible?
Lighting
Since most dealers take photos outside, whenever possible position the car so when you're taking the front shots, the sun is behind you.
Don't use a flash in the sun. Most cameras are limited in shutter speed when using a flash. Limit a shutter speed too much and you end up with blown out highlights when the lens can't stop down the aperture enough.
As someone mentioned before, cloudy days cast an almost perfectly even light over your car and typically with no shadow. Bump up the exposure about half a stop and you'll have great, saturated colors and no hotspots.
One of the greatest advantages of professional cameras (DSLRs) is their dynamic range - how bright something in the scene can be without it over affecting the dark parts. Cars are one of the hardest things to photograph - they're shiney, they tend to have really dark parts (rims/tires) and really bright parts (paint, glass, reflectors, lights...). A good camera with a lot of dynamic range will let you capture all of that.
Downside to DSLRs, they are bigger, heavier and more expensive depending on the lense - but maybe a small price to pay for the added quality.
Chip-