[Nemo]

You could start with things that you are nearly guaranteed to need:

Bumper materials (noodles, fabric, plywood, fasteners)
CIM motors
Wire
Fasteners
Jaguar and/or Victor motor controllers
Controllers (gamepads / joysticks / custom controls of your team's choice)

If your team has particular building materials that you know how to use well (or want to learn), you could have some of that on hand:
Sheet aluminum, T-slot extrusion, wood, PVC pipe, AndyMark C Channel, angle aluminum, box aluminum, polycarbonate, etc.

If your team has a type of drive that you like (or want to learn), you can buy parts for that and build one in the offseason. Then you can use any items next year if desired if they are still unmodified COTS parts. Includes wheels, sprockets, spacers, bearings, gearboxes, etc.

Strategically investing in tools is smart. Getting safety glasses that aren't all scratched up is pretty nice.

What you'd want to buy preseason depends on a bunch of things. If you have an offseason project in mind, that would tend to drive what you buy. If resources are very limited, one would probably want to get things that are a safe bet to be useful on next year's robot. If you want to have greater flexibility when you find out the game, you'd want to get tools and raw materials that can be used for a variety of tasks. If you want to get off the ground quickly next year, you might want to choose a style of 6WD (or some other drive), build one in the offseason, and have the components ready to go for the start of 2012.