The maxim is simply "if you're really interested in fire trucks, and you can afford one, then you can buy one," and there's no problem with it becoming a "universal law," because very few people will be that interested in fire trucks.

Also, if everyone were to try to buy firetrucks, the price goes up — and makes me wonder how supply and demand fits in with the categorical imperative (if at all).

Do you want to live in a world where I get to decide if your lifestyle will scale to seven billion people?

Because I'm thinking your computer would use less electricity and cause less CO2 pollution if it were half as fast as it is.

Kant was wrong.