What are the rules for 100% safe trade?

Can someone tell me how to trade 100% safely?

For example. I want to buy WAX by sending to PayPal.
How deal takes place? First the seller send his tokens to the escrow and when it’s funded I will send funds to the seller’s PayPal. What if the seller cancels the escrow? I know that’s kinda dumb question but I need to know. How deals take place to be 100% safe.