Yes if parked on a public street
I do not think it is illegal to do so but it may be illegal to drive with the sign in a window required by law to not be obstructed from view. I think in California that would include all of your front windows i.e. drivers, passengers, and front wondow. It is also illegal in some places to leave a car parked for the intention of selling the vehicle, for example if you parked the car at 711 by the entrance or in the parkinglot and you were not utilizing 711s services you could be breaking the law. That also includes some street parking in various areas.
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