This is not a true statement in itself, the assumption is false.
It is seemingly a relevant statement only because drops are raw rng and not intelligently designed in PoE.
All the problems of easy trade(and also those of inefficient trade) can be solved with the very best items being bound and intelligent drops.
For example the ones you Buy POE Currency mention -
- Easy trade reduces the number of times a character improves their items.
- Easy trade means reducing in POE Items drop rates
- Easy trade would make the disparity between different players too great
- Easy trade allows for greater abuse by automation