Digital Zooms are the camera's software guessing about how to fill in the spaces when you zoom in.
An optical zoom is done the same as a regular camera - it actualy zooms in and when you take the pictuer, each pixel will be from the image, not the camera's guess as to what the pixel should look like on the digital zoom.
Again, if you're only going to be putting the pictures on teh web (posting for EBay), you'll probably be fine with the digital zoom.