Let's say I write a book and it is selling pretty well. Maybe it brings in $300 on average a month. Can you sell that book to someone else so they can continue that income stream? I guess you would have to unpublish the book, then sell it so the next person can republish it? I would think that this would be a big loss in ranking. I don't even know if Amazon allows it.
Since it is providing a steady monthly income it is an asset. Like selling an Adsense site making $300/mo, or maybe a product site that brings in $x/mo in sales. And would this be the 8x-12x earnings range? Just curious.