Yes, you can sell them to anyone down the line. I agree that early access would be a cool feature. Investors would be taking a stake in a writer as a whole, almost like a race horse, and can help their own bottom line by acting as a beta reader for said writer, telling people about "their writer's" books, tweeting links to their storefront, providing editing, cover design, deal making. Mentorship. There's a lot one can do if you like someone's style enough to take a risk on them. Mine are for sale, back-alley way style. Free e-book pdf's available on request.

I don't think the legal needs to sell these are too complicated. A simple contract would look like "The holder of this contract is entitled to x percent of the monthly royalty payments paid to y author" and the investment instruments are instances of said contract.