The easiest thing to do is to experiment. Find the settings, lighting and backgrounds that give the sort of result that you are happy with.
Depending on your camera, you may be forced to accept fairly basic image and lighting settings or you may be able to use quite sophisticated ones. If you are lucky enough to have a digital SLR camera, then you will have all the control you will ever need (although I find that my "old" Nikon Coolpix often produces excellent results much more easily than with my Minolta digital SLR :!: )
However, I would strongly recommend trying shots using both "default" and "close-up" / "macro" distance settings. Some cameras focus very well at a 2 feet range on the "close-up" setting but others will not focus at all at that distance! [See eBay for plenty of examples of out-of-focus images most likely caused by not using the right setting for the distance.]
Another thing to avoid is taking shots with more than the target item in view. That's another reason for so many out-of-focus photos - the camera finds the best focus according to its settings, which often means the intended target is not the one the camera settles on.
And another thing to avoid is the "shot-in-a-chair" photo. So many times we see a wonderful image of the upholstery of a chair with a tiny (and again out-of-focus) view of the target item.
Get close. Experiment with the settings. And try each image with and without flash. After a few practice shots, you will find a set-up that works for you. After that, stick to it.
So much for taking the shots. What about "processing" the image to produce the best size and quality for your listing purposes? That's a whole different topic and again depends on many factors including the type of image processing software you have. But the basics I recommend for checking out in software Help info are: "Cropping", "Resizing" and "Sharpen" (or "Unsharp Mask" in many applications). Again, just like taking the photos, getting the best results for the display purposes is a case of experimentation - what's best for printing is not best for internet listings.