Pro tip for first-timers: Carefully consider the pay rates that are listed. Penny Hoarder Carson Kohler has used the platform to find freelance writing gigs, and she reports low rates — like $3 for 500 written words. It’s probably not worth it. (And, yes, you’ll have to scroll through a whole lot of these low-paying listings to find the good ones.)
You can sell your ebooks through Amazon's Kindle program or Apple's iTunes Connect, which gives you access to a large majority of the digital-book reading market and the barriers for entry are incredibly low. You don't need to invest lots of money to make this happen, but you do need to invest lots of time, not only in writing the ebooks, but in marketing them as well.

Amazon makes it fairly easy to list and sell old books, games and devices on its marketplace. You can make more than just a few bucks If you have pricey textbooks from college. Be sure the books are in good condition. You'll get negative reviews if you attempt to sell books that are falling apart or games that are scratched up. Remember, be upfront about any defects, no matter how small they might be and no matter how few people might readily notice it. 
Freelancing is the next best thing to being paid more for your full-time work, because professional work always pays more than unskilled. To find opportunities, let former colleagues or other personal connections that you’re available for freelance gigs. (Here are some ideas on how LinkedIn could be useful for that.) Or, post on marketplaces particular to your field. For instance, Mediabistro, a journalism site, allows freelancers to post profiles of their experience and services. Though these are more up to chance, designers can bid on jobs at or submit a design at Threadless, to see if it will be crowdfunded. Elance-Odesk also lists many freelance opportunities, and you can also post your own services on Fiverr, although some freelancers say these services create a race to the bottom on fees and so are not very lucrative. If you're new to freelancing, here's how to set your rates, and here's how to negotiate raises with clients.