I think you should offer them at a price that works for you. Offer them as a custom build and if you never sell one, who cares, you have nothing invested.
A beach cart and a hauler can easily cost 500-700 seperate. I am not in your target market, I bought my big foot for $150, but someone paid $500 for it new. There is a market. Not everyone is cheap and buying used.
I do agree with the problem of custom fab. I have a harbor freight rack I have used to carry my ski and it drags the driveway hooked to my Silverado. Your design would not work for me, but I have a very steep driveway.
Anyway, I am sure there is someone out there that would buy one, but you'd probably only sell a few for a minimal profit. Don't tie up anything in inventory, and it might be worth your wild.