Short version.
"If not copyrighted, NASA material may be reproduced and distributed without further permission from NASA. "If the image contains people, you would need a model release. You can't use their logos. You can't use Nasa materials to endorse a product. There's also something about people claiming copyright to materials that they have copied from Nasa websites.
Just a guess, but it's probably legal to upload these to stock sites as long as the person don't claim they are his or her images.
Don't the sites say, you must be the copyright owner of everything we upload?
Hopefully anyone who is thinking of paying for these will notice and go download them for free from the Nasa site.
