This has become a popular business model for online entrepreneurs over the past several years, and will probably just continue to grow in popularity. The best thing about selling online courses is that once you do the up-front work in creating the course and setting up your marketing strategy, you can get paid over and over again for work you do once.
If you're a teen with stuff to sell, consider looking around on Facebook for some local yard sale groups. You can join and then post your used video games, electronics, or anything else you might want to get rid of. This is a good option because there really isn't an age limit. However, definitely use caution and don't meet anyone in an isolated place alone. If you are not yet driving, have a parent or guardian drive you to meet the person you are selling to.
I have not personally done this but I would like to eventually. If you are artistic you could make artwork or handmade good such as ornaments, wallets, or jewlery and sell them online. Something that I personally would like to do is design T-shirts and sell them. Shop for anything from creative people everywhere is a great site and I have bought many things from them. If you make quality products and get your name out through Facebook or other social media you could get a substantial business going.