The new telescope delivers stunning images of the cosmos which are available on NASA's web site.
The content use policy page states:
Unless otherwise specifically stated, no claim to copyright is being asserted by STScI and material on this site may be freely used as in the public domain in accordance with NASA's contract.
Does "in the public domain" mean that I can legally make prints of those images and sell them in order to make money, for example in a store or flea market?
Yes.
I am tempted to leave the above as the entirety of my answer, but to expand:
Yes, you can do that. You won't (as a business matter) be able to charge all that much because many people will realize they can just download a similar picture from the net and print it off themselves. You can charge a bit because you've framed them (perhaps), and chosen the nicest, and perhaps you have a better printer, or have used the expensive glossy paper. Also, if you are successful charging a lot, other people will start selling the same pictures for slightly less (and you won't be able to stop them).
