Amazon pays around a 4-6% commission on the products that you sell, which is quite low compared to other programs out there. Although I sell lots of ESL textbooks and ESL teacher training type books through my blog: My Life: Teaching in a Korean University, it doesn’t really add up to any substantial amount at the end of the month because all those sales are $15-40 or so.
The things that can really make you a decent amount of money are the big ticket items, that is things that sell for $500 or $1000, but which people will generally buy online instead of in-person. I’m not going to tell you all my secrets, but I have niche on Squidoo that sells bigger ticket stuff through Amazon and that amounts to a significant portion of the $100+ I earn on that platform. I could earn even more if I developed my own websites in that niche and cut the Squidoo commission out of the mix. But, that would be a project for another day!
So, think about it. What do people buy online that costs a decent amount of money ($100+). Then, use Google analytics to see how much competition there is. The sweet spot is low competition, buying online instead of in-person and high ticket price.