There are several options for selling your car designs. One way is to create a one or two-page sales sheet that outlines the problem your design solves and the benefits it offers, and then submit it to car companies. You can also use visuals, such as renderings or 3D models, to help communicate your ideas more effectively. Another option is to sell your designs through online marketplaces, such as Wrapstock or CGTrader, which allow you to connect with potential buyers and earn royalties on your sales. Additionally, you can create an online store through platforms like Zazzle and sell your designs on a variety of products. It's also worth considering selling your car designs through specialized websites that cater to the automotive industry, such as CarGurus or Carvana.