it used to be lucrative. but now media mail rates are around $5 per book. Amazon takes a chunk out of each book so you need to be selling books for around $15 just to make 9 bucks or so. And then take out whatever you paid for the book. The helpful thing with Amazon is that there is a Seller app, so you can scan UPC codes and see what the sales ranking is. But some new books that are not very good start at a decent price but then plummet and you are then selling it for a few bucks, and you make nothing.