Introductory offering $89.50 +/-, free shipping. See what happens. The no.1 task for any invention for sale is reduce the cost of production. Make parts the same on both ends, you did that. Use stock sizes, switch to aluminum if cheaper or easier to machine, reduce number of parts, add buyer work content, purchase parts/material in volume. Cut many parts at once, make jigs, no measuring, coupons, put an ad in Stovebolt like Deve did. Write to "Sharks". Just thoughts, not expert advice. They will sell, but not like hot cakes. It's a neat, novel, easy DIY and hopefully affordable Custom upgrade. Make sure the name/phrases you choose self identifies the part. Tailgate Chain Eliminator is great. "Hidden Tailgate Support." "Easy Install".
NEW! is the number one word recommended in advertising.
I'm not your Daddy but don't get too invested yet. If it only takes 4 hrs, you can make to order for now. Don't need big inventory of units or material, yet. You will get very fast and efficient at making them. Find some perfect size cardboard tubes at a carpet store. Find a jar cap that fits the ends. Backwards, inside. Use USPS online printed labels, saves money, they pick up for FREE. They are the cheapest way to ship for now, it does change from time to time to FedEx, never UPS.
Good luck Dave.