The thing is, there is already a way, which is to get a paypal account and embbed the paypal buttons in the page for your image.
Here is an example of how that looks:
http://www.pbase.com/53digital/image/91531932The problem is having to manually add that code to each and every image you upload. As far as I am concerned if they came up with a way of being able to include that in the upload process I would be more than happy.
My suggestion would be each image has an option to determine whether it is for sale or not. You can select that on upload for the whole set of images you are uploading.
You have a separate 'selling' config where you determine sizes, options (mounted/unmounted, matte/glossy) and prize.
This config can either be general, or applied to specific galleries. (just like styles).
Et voila.