How do you collect bat guano from a residence and sell it?
I don't think you can do that. Guano can become toxic if left
for too long, which is why many homeowners have it removed. For it
to be useful, bat guano has to be fresh, kept away from light and
moisture, and can't have been left for too long.
And it is sold by the ton, something you are not likely to get
out of a house.