Not every 3D model will be popular. The customers that buy from CGTrader are diverse. Some are buying stuff for gaming, others for advertising, and some for printing. You can look in Freelance 3D projects where customers look for particular things. Generally, you should model stuff you are passionate about since that will most likely show through your work. In addition, don't model something that has been overdone or has several iterations of it, as buyers will have a large selection and may not choose your work, because of detail or cost.