You can sell other people's items as an affiliate marketer. You register with a site such as commission junction or shareasale and use your own website to promote the item. You receive a portion of the sales generated by your promotions or a flat fee for registrations and purchases.
You can use PPC advertising to promote someone else's product if you don't have a website of your own.
You can put text and banner ads on your website that are contextual - meaning that they are related to the topic of your site. All three of the major search engines run programs. You earn a small amount of money each time someone clicks on the ad.
You could raise a hefty sum on eBay if you already have things that you want to sell. You could also find items cheaply at garage sales and then resell them on eBay.
You can join the free online survey sites, but don't expect to make a lot.
Write your own eBook and sell it online either on your own website or through a site such as clickbank.