If you have a scanner try scanning coins. I find this is easier to get nice clear images than with a camera.
Try different backgrounds -normal white backsheet of scanner or dark grey or black. I prefer the darker colours as this eliminates shadows around the edges of the coins. The following scans are as they came from the scanner with basic resolution and colour settings.
As ebay picture sizes are not large 300dpi scan is usually more than adequate and lowest colour resolution is usually fine. If you get problems with colour fringing -the better the coin and the finer the detail the more likely this seems to be- then try a grey scale scan, or convert to grey scale when saving the image.
Any reasonable picture processing program is fine to adjust brightness, contrast, crop to size etc. I use an ancient version of Picture Publisher (I am used to it) and only problem I have is it will only save to old style DOS 8 letter file names.
Also check out your scanner software control. Selecting Custom Scan instead of using default settings will let you adjust colour, contrast etc. directly from the scan and might avoid fiddling about later to get it looking just right. Have a play with options like "smooth, remove pattern, sharpen, contrast/brightness" etc. Your software might call them slightly different names but most have similar options for image processing.
I hope this brief guide is of help.