My question is similar to the original poster's question, except for that I'm trying to understand what size to save a photo in if you're uploading it as a high resolution photo for print. Would you still save it on the low-end resolution as "8" in Photoshop or up it to the highest resolution as "12"?
Always use the highest (native) resolution on your camera, and after editing keep the photo at its native original resolution.
You do not need to, and should not, reduce the resolution simply because you plan to sell the image in future. Most people who buy the image would want it in its original resolution, and if they don't, they can do the resizing themselves.
Only when you are told you need to reduce the image to a specific resolution for a specific purpose should you create a separate copy of the image in that resolution.
As for file format, firstly, always keep the source image, as it came from your camera, unmodified. That way if you screw up you have something to go back to. When storing an image for your own use and editing, use a lossless format - it might be a good idea to use the native format of your editing software, eg PSD for photoshop. When delivering the image to someone they may specify which format they like, or you can use JPEG with a high quality setting. For long term archival use something lossless but well-established like TIFF, though PSD is established enough to be supported for a long time too.