If you go to Amazon they sell these anti-counterfeit pens for under $10 that you swipe over the money to tell if it is a counterfeit bill. Make sure you report this to the police and report this to the Care team here. If this buyer gets caught using these fake Bill's, they would be in big trouble. Also in the future when you meet with someone who drove to meet you--jot down there License plate# along with the car they were driving. Good luck.
Agui1 I would highly recommend reporting this to your local authorities, making a report with them., also reporting this issue through the persons OfferUp post taping on report., and giving as much information you can., also taking a screenshot of the persons profile page for your records.,I’ve also put a link below to customer care if needed, 😎🇬🇧🔚
I had a similar experience when selling a laptop. Buyer game me $320 in fake bills. As soon as he handed it to me I suspected they were fake and told him so. Then I used my counterfeit pen to check. Never confront the person just play dumb like they might have been given the fake bill by somebody else. You never know how they may react. When accepting money you should always assume it could be fake. Alsways.