Use Simple, Inexpensive Lights for Large Items
Lighting is one of those disciplines that seems super complex, but for most things you'll ever need to do, it's incredibly cheap and easy. If you're selling a sofa, a desk, or something large, you create effective lighting with simple clamp fixtures, compact fluorescent bulbs, and wax paper. This is also a great time to brush up on your three point lighting (though for most things you're selling online, the key and fill light will probably suffice).
If you're not keen on having a bunch of clamp lights laying around that you only use when taking photos of things you're going to sell, flexible floor lamps work pretty well, too. Cheap lamps can be picked up from most large retail stores and, when not in use, they can be used as part of regular room decor.
Build a Light Box for Smaller Items
Light boxes are incredibly simple to make and create professional-looking photos that highlight your item. Of course, you don't need something quite as elaborate as the photo above. In fact, you can make a box out of poster board, cardboard, or even a bucket. If you're willing to do a little work, you can make a box for under $10.
Be Mindful of Your Surroundings
Few things can ruin a well-composed picture quicker than a messy background. For things that don't fit in a light box, be sure to clean up anything within the frame that may turn off potential buyers.
Additionally, it may seem unnecessary and silly to say, but always wear clothing while taking photos. Even professional photographers can miss stray reflections, and you don't want to end up getting a ton of views for something other than the product you're selling (NSFW).
Selling things online is a worldwide competition and there's a lot of noise for buyers to get through. Taking better photos isn't the only way you can improve visibility, but your potential customers will want to see the product. The more high-quality pictures you can take, the better.