How do I report a seller, a buyer or a listing?

There's a report control on a listing, on a person's profile, and inside a conversation. Use whichever one is closest to what went wrong.

It goes into a queue a person works through. You won't always hear how it ended, because what happens to somebody else's account is between them and us, but every report gets read.

If money is involved, this isn't the one

Reporting a person and reporting a problem with a deal are different buttons. If you've paid and something's wrong with the item, use Report a problem on the deal itself. That's the one that freezes the money.