Both of these are solid and valid points, other games have solved this problem.
While some of the item do in fact perish when a player quits or decides to NPC an item (which the OP could simply buy all the NQ and NPC them if she was so inclined to "fix" the market)....but this is rare.
Other games solution is to make some weapons/armor "customized" once equipped , or for WoW players "binds on equip"; meaning once the item is used by a player it cannot be sold. I know a lot of you may think that's kinda harsh for the buyer, but it's really not that bad because prices are much more stable. IE: only those who NEED the item buy it usually. Once the item is no longer useful it can be sold to an NPC (taken out of the system) for a FAIR price (not a total slap in the face) allowing players to recoop about 50% of what they should have paid for the item; which in turn allows them to purchase a newer weapon/armor from the crafter. It's a fair, and useful system; that makes sense. (How can a Roegedyn, buy, re-use, and wear the same acton that was customized and worn by a Lalafell?)
Again, as with most of my posts I'll mention.....this isn't rocket science....these methods to combat the problems the OPer and other see have been around for a decade already. SE just didn't bother to do their homework.
EDIT:
I'll also add in that the only way to have a stable market is to make most every item/weapon/armor available at NPC's for 5 times the crafting costs (or a fair price for base mats) thus making the player sellers unable to over inflate the price of that item, yet keeping the crafting profitable when the crafter sells the item under the NPC price. In turn players prefer to buy from crafters because they should get a steep discount on the item by doing so.
If a player base attempts to over-inflate the price of a base martial, rather than allowing the trickle down exponentially inflate prices of crafted items; the crafters simply go purchase the item from the NPC until the player base realizes no is gonna buy that item at such a ridiculous price.
Again, this isn't a new idea, or speculation, or one that "may or may not" work....it's a system that has been in place in many MMO's with success for quite a few years.