Running a classified ad platform is not that easy because fraud is everywhere and we are using cryptocurrency. its easy if its PayPal, My suggestion is to encourage the use of escrow. You can be one or ask a reputable platform to do it.
Another one is to have a page that is dedicated to educating your users about fraud and scams and to always check if there are complaints and report, you have to be active if you want to gain the trust of your users.