Is there any market for a rabbit manure based soil?
Barely a market for rabbits to begin with. But in my opinion rabbits and their "waste" provide one of the best soil amendments. I've been using them for 10 plus years
I’m guessing it’s a smallish market, but I’ve bought some rabbit manure on Etsy before. It is good mushroom substrate.
I had a client try exactly that about a year ago. He decided to do something else, but its all in the marketing, you could succeed. I'd suggest you need a direct line to local people who are into gardening (FB? Instagram?), a good, flexible, low-cost system for packaging and delivery, and a lot of patience.