It's about the same risk as pre-registering for an offline tournament with people you don't know. Trusted sites and online TOs can be trusted as well plus I'm not just a random guy trying to scam look at my post count on here.
Also this upcoming tourny Im planning to have 100% of the entry fee go to the pot with no fee at all. Soo that means no venue fee no TO fee. It will all go to the pot.
Except if the TO runs off with the money from an in-person event, you more than likely have a name, a description, and a location you can give to the cops, as well as some more info if it's at a rented location, like an arcade.
You give your money to someone on PayPal and they run, there's not a ton the cops will do unless you're talking thousands of dollars in fraud, or you already know exactly who the person is.