1. If you do not own a tripod, and not many people do, then try to stand your camera on a hard service to avoid shake from a freehand shot. This could be as simple as a stack of books for a tabletop shot.
2. Think about the light around your item - a light from behind will silhouette your item, so details will not show.
3. Daylight will always give a truer representation of the item than artificial light.
4. Try to keep peripheral clutter out of shot, this will distract from the item for sale.
5. Take at least a couple of shots if you can, from different angles.
6. Think about staging your shots - no need for anything fancy, but the "look" of pictures can influence how people feel about you as a seller, and therefore about your items.
7. If the item has wear, scratches, defects or other blemishes, show these in the photos - honesty is always appreciated, and if it does not detract from the overall use/look of the item then buyers will be able to gauge and appreciate this.
Believe me, a good photo WILL make all the difference if you are serious about selling - personally, I won't buy an item EVER if the photo is blurred or too dark (or too light for that matter) - you just don't know what the bad quality image may be hiding.