How to report a scam

There's a 2002 Toyota green truck for sale and I know it's a scam because I looked into it almost bought it myself and I verified it through eBay motors and they told me it was a scam and did not come from them how do you report that because I posted on it that it was a scam and he blocked me and it says it's pending right now I just don't want someone to fall for it I can't stand the scammer!