Clearly, there's a lot of demand on Amazon, and if any product is going to sell, it's going to sell well on Amazon. But the goal here is to source the right products that will easily sell at the world's largest online retailer. Generally, products between $10 and $50 sell very well here. Just be sure to do the right market research before jumping on this bandwagon.
…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.
I see a comment made by Stella including some freelance sites. This does work. I use oDesk and it’s been a big help in bringing in a little extra money. I also agree with some of the other comments that mention people should think of or view more ideas of making extra money, because it really is possible to make extra money instead of just sitting around moping about not having money to pay the bills or buy something special.