Clients usually make a first contact with a message making you aware that they are interested in doing project with you. After both of you agree on terms of making the project, client will hire you and prepay the money for project. Prepayed funds will be released to your account when you finish the project.
Sometimes it happens that even the client doesn't know how the system works. I think it was my first or second project... I delivered the product, client thanked me and didn't mark the project as finished to release the funds. In such scenarios contact the support. They will investigate the case and release the funds in your favor if they see the project was actually finished.
The system was made so that both client and worker are safe to get what they agreed upon.
If you don't want to be tricked, do not ask or give e-mail address to potential client and do not do the projects outside CGTrader site. The fee that CGT charges for projects is your security that you will be payed if you finish the project properly.