If you're ready to enter the ecommerce fray, you could sell your own stuff. Of course, along with selling your own stuff on your own website comes a whole slew of both responsibilities and technical configuration and requirements. For starters, you'll need a website and a hosting account. You'll also need a merchant account like ones offered by Stripe or PayPal. Then you'll need to design that site, build a sales funnel, create a lead magnet and do some email marketing.
The biggest advice I can give is to pick a small niche, set up a site and start experimenting. Make mistakes and learn from them. That’s the only way you are going to succeed. Of course, have realistic expectations. It took me more than 1-2 years to start making decent money. But the beauty of it is that once you make that first dollar, you know it works and at that point it just becomes a matter of scaling it up, working harder and smarter to scale up your business.
…Marketing an eBook is definitely the hardest part. Ebooks have exploded in popularity and the market is unfortunately saturated with crap from people who thought they could write a quick eBook, put it on an affiliate network, and make millions. However, if you CAN find a good way to market (or possibly have others market by having it listed on affiliate networks) then an eBook can be a great way to make some residual income.