When one of your customers makes a purchase, you purchase the product from a third-party company (the drop shipper, usually a manufacturer or wholesaler) for a lower price. This process is as simple as forwarding the order from your customer, a process that can actually be completely automated. (Remember you don't have any risk here of buying inventory because the sale has already been made).
Fast growth usually ends badly for most businesses. Fast growth may not be manageable without the right people. Build for the future. Target small areas where you can avoid big encounters that are able to take you out of business in just a few minutes. Remember to research those ideas, make sure that the idea is feasible, and try to find a niche product with low competition and high demand, preferable something you are already familiar with. Then start implementing the idea by starting small and continue with slow and stable growth. Long term earnings are always greater than short term earnings.
Shopify is a great example of the freemium model. By offering some applications for free, they get exposure for their whole business. Since the company was founded in 2004, they have acquired 30,000 customers, including Pixar and the Foo Fighters. Their revolutionary “build-a-business” competitions in the past have produced over $70,000,000 in revenue for over 14,000 entrants.
One of the most important internet marketing strategies is to develop every customer's lifetime value. At least 36 percent of people who have purchased from you once will buy from you again if you follow up with them. Closing that first sale is by far the most difficult part -- not to mention the most expensive. So use back-end selling and upselling to get them to buy again: