This problem was not caused by cross-world buying. Yes, some mats don't sell well on the marketboard. Find the ones that do, and farm and sell those. There are plenty that are sub-80 - and even sub-70 - which sell for a pretty penny still. Elementary MMO money making.
Low prices are good, because it makes DoH classes easier to level, which in turn allows people to produce items that are worth something. A player not doing research into what they can capitalise on to make money is not an issue caused by cross-world trading.
If only there was some kind of measure through which it could be stabilised; something through which prices could reach something of an equilibrium. Also, the problem of mats selling for vendor prices was not caused by cross-world trading.
They should, because it has not as of yet had a demonstrably negative impact.