If you have some good writing skills you can write a book and sell it online. The effort needed in writing might be huge but it is a onetime effort. Once you are done writing it, get it reviewed and create an eBook.
You have plenty of sites that let you sell eBooks. For instance http://www.ebooks.com/
If you have programming skills, you can create and sell your software applications on Share It
If you plan to sell directly from your website then you can choose PayPal's website payment service or Amazon's flexible payment service to get paid directly.