If your blog or website is visited by peoples who can buy goods, but you don’t have anything to sell, you can promote products and services what are for sell by others. Almost all online stores have a affiliate program so, if you recommend their products using a link provided thru such program, you will gain a fee from the sells generated.
A good affiliate website can make more money then a traditional store, because he don’t have the same amount of expenses.
However, it is not so cheap to make such a website. It is very difficult to ad the content, who must be generous, and to promote the website.