My guess is that the original Mavic Pro has been out for a year so owners are:
1) Upgrading to a Platinum or,
2) Upgrading to a unit with better camera or,
3) Have flown for a year and the magic is gone
or,
4) Are fed up with DJI's crappy software, customer service, and general business practices.
In my opinion:
1) The new low noise props will get you close to a Platinum and,
2) A professional photographer may need a better camera but the stock Mavic camera is pretty good and,
3) You need to find new places to fly, check out other software for new features, get involved with forums like this one
and,
4) There are ways around DJI and their software.
There are undoubtedly many other reasons people would sell their Mavic but, for me, I am in it for the long haul. The Mavic is truly a marvel that is worth putting up with the negatives.