Yeah, I've sold ~$100k (gross revenue) on Amazon over the last 2 years. My college roommate had gross revenues of $900k+ for 2016. He's basically doing it full time. I've more or less quit. Two reasons I've basically quit are 1) I had a 3rd kid, and 2) I've started traveling too much at my day job.
There are a few different models out there for amazon reselling. Four big ones are
1) Retail Arbitrage - go walk in a store, buy some stuff, resell at a profit on amazon
2) Online Arbitrage - go to a website, order some stuff, sell
3) Wholesale - develop relationships directly with brand owners, develop the ability to offer them value propositions in return for discounts, semi-exclusive, or exclusive partnerships--e.g. I can help you improve your amazon listing and walk your company through the brand registry process in return for me being the only person authorized to sell your product on amazon
4) Private label - create your own product and sell under your own brand. A lot of people do this by ordering from alibaba.com (e.g. I could by 1000 instant read thermometers from alibaba.com for about $4.50 each, slap my own branding on it, and then try to sell on amazon for $20. Problem is, a dozen other companies have the same exact thermometer with their own branding on it. Can be very competitive.
I would say in the two years I've been "in the game," the game has gotten exponentially more competitive. When I started, it was almost too easy to find replenishable products. I basically did it by hand (e.g. no software). These days, I think software and various plugins to make sure you are finding the best products, prices, and stackable deals is a necessity.
If you have time and enjoy spending a lot of time online (or want to go the wholesale route and really enjoy the service industry), it might be a good idea, but I find it harder and harder to spend a few hours a week and make a little extra cash on it, which is why I started.