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The Facebook Marketplace has a simple, three step process for reporting a listing that you believe is a scam, or that otherwise violates the Facebook Marketplace Community Standards. Go to the Marketplace and find the item you suspect is a scam. When you click on that post, you'll see a link that says "Report Post" on the bottom right. Click that link and follow the instructions to make your report. In the United States, you can report a Facebook Marketplace scam to the FBI using the Department's Internet Crime Complaint Center (IC3). You can use this service if you live in the U.S., even if the scammer is elsewhere or you don't know where the scammer lives. If you live outside the U.S., you can still file a report if you have reason to believe the scammer is located in the U.S.  Go to the website at https://www.ic3.gov/default.aspx to learn more about the service and file your report. The information you provide will go into a database that is used by federal, state, and local law enforcement to identify patterns of fraudulent activity. Gather all the information you have about the person who posted the scam listing as well as the listing itself. While filing a report with the FBI does not mean that law enforcement will actively investigate your case specifically, it does help their efforts and may lead to additional evidence that helps stop the scammer. Especially if the scammer appears to live in your local area, filing a police report may help authorities handle the situation. Keep in mind that someone who attempts to scam one person will likely attempt it again.  If you've already reported to the IC3, you can provide that report to your local police. Bring all the information and documentation you have about the transaction, including a print-out of any conversations you had with the scam artist through Facebook Messenger. Go to the police station in person to file your report. Do not call 911 or your country's equivalent emergency number unless there is an actual emergency and you feel your life or safety is in immediate danger. Get a copy of the police report for your records. You may want to call the officer who filed the report after a week or two to follow up if you haven't heard any news on the status of your case.
Report the item to Facebook. File a report with the FBI. Contact local police.