You’re Too Helpful
People are trained to work in specific departments. That way, they can gain skills in that department and climb the corporate ladder. However, if your business is just starting off, many parts of your company will probably be managed by the same people.
So, the OP’s boss wasn’t too pleased when they, who worked in sales, manned the cash register one day. That actually sounds like a real go-getter to us, but the boss didn’t think so, and they fired the OP for being too helpful. Maybe the boss should’ve scheduled another cashier to cover that shift?