[AllenGregoryIV]

So much of this depends on your teams building methods. We can recommend what we use but if you don't design the same way it's useless to stock the same things.

However what we have used most often is:
1/16" Sheet Aluminum for gussets
1 in L aluminum in 1/16" and 1/8" thicknesses
1 in square tube aluminum 1/16" think (with bumpers we find you don't need more than that, and its so light)

1/16" polycarb and a small amount of 1/8 when we need something thicker

1/4" HDPE it's useful for all sorts of things and has a low coefficient of friction which is handy some times. Probably could go thinner

1/2" aluminum shaft (keyed and unkeyed)
1/8in Keystock

Bushings, Bearings and aluminum shaft collars in the same size as shafts that you have.

Sprockets, chain and links are really useful to have on hand.

Fasteners, we use 3/16 rivets and #10-24 bolts for most things (you can get away with using a 13/64" drill bit for both of them so it makes things easy). Lots of lock-nuts.

Zip-ties

Surgical tubing is always nice to have and polycord is useful for games like this year.

Lots of wood and PVC for prototyping (and cardboard but that is free)

For electrical, I would also stock up on wire, common sensors, and connectors (we use Anderson Powerpole connectors everywhere)

That is most of our standard stuff, everything else is game specific.