Honestly- the simple things can rack in the money super fast. Auctions are the best imo.Here is an old guide I made to making rupees. The info should be relevant still.
Are you that african prince that asked me for money in that email????? It worked!!In all seriousness mining quartz ore or trading with villagers is your best option. Trading with villagers for enchanted books, emeralds or whatever is the effortless way which just takes up a lot of time and is SUPER boring, but profitable.Mining quartz is dangerous but very worth it in the end. Just bring an ender chest, fire resist potions , ender pearls and you're set.