I sold a phone on craigslist now these people want there money back?

So basically I put up a Droid 2 thats in like perfect condition but with bad ESN. I would of took any offer. So this couple offered me 100$ so I met up with them they checked it out and liked it and purchased it. So the next day (today) they are calling/texting me asking for their money back because I sold them a phone with a bad ESN. In the ad it Specifically said this phone had a bad esn that's is why I was taking any offers..I was just wondering can I get in trouble with police or anything? Should I give there money back or is it the buyers fault for not reading the ad?(BTW the phone is NOT stolen). What do you think? http://rockford.craigslist.org/mob/2348543529.html there is the ad its self.