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.
Crafting cosmic runes in the abyss is a great way to make gold through the runecrafting skill. To start off, all you need to do for this method is fill your inventory with as many pure essence as you can, filling your runecrafting pouches and summoning familiar till the brim, and run into the wilderness. You then need to make your way over to the abyss and enter it. You then need to enter the inner circle and find the cosmic hole and enter it. You then enter the altar portal and craft your runes. You then teleport to the Edgeville bank and repeat this process. A more detailed guide can be found in the video linked below. Using this method, you can easily get upwards of 2 million gold per hour with relatively low stats.
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.