This is a very subjective area....what makes one photograph saleable and another with no apparent resale value. The price of any item is dictated by how much someone is prepared to pay for that item. I personally would not even accept a free Ansel Adams print as I just don't like his work, I find it devoid of any emotion. However I love Bresson's work and if the chance came up to own one of his prints then I'd jump at it. But hey that's just my taste. I'm sure that Magnum continue to do very well out of sales from Bresson's work just as the Adams estate do very well out of Ansel's.
On another level I do think its one of the great tragedies of our time that most people buy art as a disposable decorator item and the moment it no longer matches the decor out it goes.
Photojournalistic or documentary images have a value that is beyond the pure commercial one, they are a record of our life and times. Speak to any museum curator about photographs and they will jump at the chance to acquire them. The great fear at the moment is that now digital has become the pre-eminent method of recording in 20-30 years time there will not be any record of this period due to the fact most people no longer have photos printed any more. A colleague of mine has a nice little side line supplying the National Museum with black and white fibre based photographs of perfectly ordinary day to day items. Do they have any value? Must do as the museum is prepared to pay a reasonable sum for each print.
I would suggest that the OP visit Michael Reichman's (of Luminous Landscape fame) gallery and studio which is based in Toronto. I'm sure if Reichman can turn a buck in Toronto then anyone with the right aptitude can also accomplish it. The link for the studio is:-
http://www.luminous-landscape.com/about/gallery.shtml