You'd be surprised at the market for dog poop. It can be used as organic fertilizer, as a weapon, as a training aid (for training puppies). Seriously, I've purchased a bottle of dog pee on amazon for $7.99 with free shipping via prime. I know this sounds crazy, but I'm dead serious. I used the dog pee to teach my dog where he should go. But I wouldn't have known that was even possible without marketing.

Almost anything that you think is useless has a way that it can be used that justifies someone paying for it.