Prints, maybe?

First, ensure you have the photographer's permission to sell prints of their work. Also, get that permission in a written, signed form so that you can prove you have permission when you go to get the prints made.

Then, find a company either locally or online that has a good reputation for printing photos. It might be tempting to go to places that are cheap (walgreen, meijer, etc) but if someone is paying for a print, they want good image/print quality.

Before getting a dozen or so prints of something done, get one made so you can check the print quality as well as the image quality. Something that looks good on a monitor doesn't always enlarge to a print as easily as you might think. So be sure it looks good before committing to multiple copies.

As for picking images that might sell...that's something we all wonder. If we knew what people would buy, life would be infinitely easier on all of us. My best suggestion would be to have an etsy page to showcase the images as well as conduct sales. But make the prints as they're ordered. If any images don't sell after a few months, take them down and rotate in fresh ones.