If you want to charge for images, you should only put low res images on a CD (if I'm understanding what you're asking). That means 800x600 or 1024x768 sort of size.
If you want to give them something they can print hi res photos from, native resolution is what you need.
Totally ignore dpi. It's irrelevant as to what you can print. It's only number of pixels that matter.... HTH