Amazon is a great way to unload used books. But, you can also sell a variety of other items on Amazon as well. You could opt to start an Amazon FBA. You could source products on Aliexpress and drop-ship them from Amazon. That gives you a near-hands-free method of running an ecommerce business without much of the hassles of personally dealing with returns and customer service.
Level 30-60: Cut Willow logs. Willow logs are fast exp, but their locations aren't normally quiet. You won't make much money with these trees, but enough to buy a Rune Hatchet (requires 41 Woodcutting). You can get magic seeds worth up to 150,000gp from birds nests here. Skip cutting down maples. These become available at level 45, but they are slow to cut down and not as good money per hour as willows.
Begin to kill the flesh crawlers. After awhile, you'll notice that they are dropping tons of herbs, runes, ores and very rarely they drop uncut sapphires and diamonds which are worth quite a bit. Pick up all fire runes, all nature runes and only the ranarrs, kwuarm, and other high leveled herbs (if you have the herblore level to do so, you can pick up the lower level herbs to clean them for the exp then drop them). After you have over 2000 fires runes and 75 nature runes, you should have a full inventory of herbs.
Have a yard sale to sell things you no longer need. Choose a day or a couple of days to have your yard sale. Advertise it in your local paper and online, such as on social media and classified websites. Then, on the day of the sale, arrange the items on tables, blankets, shelves, or in other ways in front of your home. You can arrange the items into groups by price, or price them individually.