If everything that you said above is true, then it's total scam and you have to require fair compensation from cgtrader. If your request will be denied, then you need to find a lawyer and consider to sue both, your client and cgtrader. However, i find very hard to believe that you told the whole story. First of all, when job is started, client can't change the details - system does not allow this. Another suspicious point - when job is cancelled, designer don't get rating for it, but when i checked your profile, i see rating for the job. Two free sales for compensation, sounds like a joke, it's really hard to believe that cgtrader could go so low.
I don't want to acuse you in lying, but we need to hear from both sides. Since you laid very serious accusations, i think cgtrader should step in and tell their version of the story. Clearly this can't be left like this. I've done more than 100 jobs on cgtrader and i have big trust and respect for their management. If what you told above is true, even if partially, i would feel very concerned about my future freelancing through cgtrader.