other than fickle minded people (which there are plenty of such ppl around in this world), there's actually one good and real monetary/economic reason why some ppl appear to buy and sell so quickly: to avoid paying high sums for rental of equipment.
take for example u need a 1dx for 2 weeks & the rental is $200 per day. so over 2 weeks, u need to pay $2800 for the camera - which is a crazy sum of money since it's just rental. some more u don't even know if the rental unit is spoilt by the previous user.
but if u buy a brand new unit, u are assured of 99.9999% working unit & brand new. You could use it for 2 weeks and then sell it off in the market for perhaps just $100 or $200 lesser.
effectively, u only pay $100 or $200 for 2 weeks rental of the 1DX. economically, this is the best solution.
there is risk to this strategy of course: what if u cant sell away ur camera? but if u buy only the popular brands and popular models, such risk is almost as good as non-existent.