Just shoot them a message like you're trying to help them out. Say, "Excuse me, I mean no offense, but the animal you are selling is critically endangered and illegal to sell. If you're reported to Fish and Game for selling it, you will be prosecuted for violating federal law, and it will not go over well. My advice to you is to take down this post and pretend it never happened."
I've actually told a person it was illegal to sell one and that he could be fined after posting one for sale on craigslist. I expected a rude reply, however to my surprise the person was very thankful as he did not know, nor wanted to be fined.
This seems to be a very common occurrence, unfortunately. A few months back, a friend of mine sent me a link to a local craigslist ad for a "sulcata" tortoise for sale. He thought it was a misidentification. Turns out, the animal was an older, probably wild caught Texas tortoise. I sent the seller a message alerting him to the fact that he was breaking the law. Paranoid guy I am, I figured I would get some cop trying to sell me the thing. Instead, I get some teenage kid who was sold the tortoise by some guy claiming to be a breeder, claiming this obviously adult animal was a captive bred baby. If the kid was telling the truth, these are some lousy people.
' I am certain that you are a law-abiding citizen and are completely oblivious to the law that states it is illegal to sell this variety of tortoise.
i would hate to see you having to pay a substantial fine or face time in prison. Just thought I'd let you know.'
Do NOT mention it's ok to give them away for nothing, or they'll just think you're a scammer after a free tortoise.