Here are short instructions you get from CGT when you apply for Project:
"GET HIRED - Client’s happy with the offer? Sit back and WAIT FOR PREPAYMENT CONFIRMATION.
Client adjusted your quote? Accept the changes or make a Counter Offer in the top right corner.
GET STARTED - ONCE YOU RECEIVE THE PREPAYMENT CONFIRMATION EMAIL, START THE WORK KNOWING THAT YOUR PAYMENT IS SAFE with CGTrader's escrow service.
GET PAID - Completed the Project? Send your client the final deliverables, so they can release your payment."
Shortly:
1. Apply for project
2. Get hired
3. Client prepays the funds
4. You start working...
Once the funds are prepaid, you are sure to get the payment when you finish your work.
DO NOT SEND FINAL FILES IF THE PROJECT ISN'T PREPAID.
The rules are quite simple and you won't be scammed if you follow them.