What is a useful anti-scam system that a small ecommerce website could use?

I'm developing a site where people can sell video games among themselves but I have no idea how to put in a system to prevent scams. So far, there is nothing stopping someone from selling someone a 60 dollar game and sending a bucket of rocks. I'm developing it solo and it would be overwhelming to have to verify every order by hand. Is there any system that I can implement to solve this problem?