Our team built a vex robot out of just 1 kit and a few custom made pieces. It was used for demonstrations and it shot small marbles in to a custom built goal.
http://www.avivrobotics.net/gallery/...ewl=417&ord=38
It is basically the regular square bot, with an aluminum profile used for marble storage (but you can use tube or anything that can hold marbles). Then we had a servo connected to a small aluminum square that opens for half a second and lets a ball fall onto the track (bent aluminum strip), then a bar connected to a motor turns around and hits the ball a few feet away. Very simple design, but it does the job.
Edit: By Blake's description, we have very similar designs. I think this really is the easiest way to built a ball shooter.