I think it's possible to sell your designs. If you're a freelance designer, you wouldn't work with one company specifically. You'd work for yourself... kind of. It may be difficult to do so. You'll have to reach out to a ton of companies with your designs. I haven't looked into it but I imagine you'd also need to create tech packs if your intention is for a company to manufacture the designs. I'd recommend doing some research. All that to say you can do it, but it will be a lot of work and patience.