The minimum tread depth in Florida is 1/16 of an inch. Below that, the vehicle will not pass the Florida vehicle inspection. I can't find anything that prevents them from being sold or limits their age but selling a tire knowing it is intended to be used on the road yet is illegal might be considered some kind of fraud. It would clearly is unfit for the intended purpose.
Under Florida 316.610, a police officer may stop a vehicle that appears to be unsafe for any reason and do an inspection. If he/she deems the vehicle to be an immediate hazard, the vehicle can be removed from the road until the repair is made. If not, a citation will be issued and typically the court will require the repair as part of the adjudication.