There's been a few threads about who needs permits in Florida.
Yes, as far as I've been able to determine, you need the permit to sell ANY bp. even if it's "just one" or if "u couldn't keep it".
Where did you advertise the snake for sale? How many snakes have you advertised that way in the last year(whether you sold them or not)? If you used craigslist, they probably trolled you up and nailed you that way.
If you go to court, be prepared to tell them the truth about if you've sold other snakes if you contest it. They may have saved the other ads or whatever they find online about sales or potential sales you've made or offered to make. Otherwise, it may be easier to just take the fine and apologize and get the permit for the future.
I get the permit if I'm going to have BPs to sell. Sometimes I've even been asked for my permit when I've sold to a petstore. Other than the stores, no one has ever asked, but I have it if I'm going to have babies to sell.
It's a bit of paperwork and yes, it's "just a ball python" but to the FWC, it's yet another exotic snake that could be an issue if they were released in South Florida.