It's a good question .. I'm thinking of getting rid of all my 1/400s - and some of my 1/200s - they just take up too much space.
I don't think there's a which scale is more popular issue - it's more of a how much $$$ do you have to spend, how much space do you have, do you collect a specific airline, specific airplane, etc. I think both scales have their fans - and then there's the 1/500 group too.
Ebay is a good option for selling - though that can be pretty time consuming. There's also a diecastaircraftforum (dotcom) - which has message sections about 1/400s, 1/200s plus a buying/selling message section.
My interests kind of cross yours - 1950s/1960s/1970s .. local service/regional .. Eastern, American, Braniff, Northeast .. I even have some of those fantasy Braniff A380s .. maybe it's time i crack open some of my boxes and see what's inside ..