S3 IS won't focus

I own S5 which is virtually the same.  The Macro mode is famously difficult to get focus from - back when there was an active forum for these models users were generally advised to avoid it because of the problem you seem to be having.  Supermacro is wonderful within the focus range indicated in your manual, which may or may not be too close for what you are doing.

With 12x zoom you should try simply try mounting on a tripod, switch off IS, back off and frame your shot as you want it using telephoto in a regular mode.  Use autofocus or manual focus, and note that depth of field can change with different zoom and different Av settings.

One more thing: since you're only shooting for ebay/web use, remember that you will reduce the resolution of your image for posting there, and so you can significantly crop a telephoto image down for a closer view, if you find you're getting better images from a distance.