Wpaint can be used by anyone for any purpose, for free. If Wpaint allows you to do something, without asking you to pay, you can do it, and you don't have to pay. All your work belongs to you. You can sell the work, which you made in Wpaint, without giving us any share. You don't have to mention that your work was made using Wpaint.
Files, which you open in Wpaint, are never sent anywhere, they never leave your device. They are processed completely inside your device, by your own hardware. Because of this, if you close Photopea without saving your work, your work will be lost.