in regards to purchasing from vendors:
that is true. but at the same time that ruins gathering jobs for a fairly long period by making them unprofitable. And from what i gathered, gathering jobs are meant to be a "main" job in the game. Further vendor items for crafting and use only create an inflationary cap. Like once the prices of goods exceeds the price at NPCs people will buy from NPC vendors, some money will exit the economy, and it will be viable for players to sell goods just under the NPC prices once again, in which case money will stop exiting. As a result the economy would average out at that point.
Put another way, 1 week you would be buying stuff from NPCs, then the next week from the wards, then the next week from NPCs, then the next week from wards, etc. NPC prices will determine the "limits" of monetary expansion, but it will not fix an inflated currency.
And that will still make it very difficult for new players to catch up because NPC items are overpriced on purpose.
One solution i can think of would be to release really good items that are sold by vendors, or have a dynamis-weapon type thing and sell the currency at vendors. But the problem still remains that a lot of people who got like 10-20 mil at the start and quit temporarily will come back and constantly cause inflationary spikes.
Also on a note: I think SE attempted to drain gil from FFXI via NPCs in CoP for end game gear. It didn't seem to work well and again, the economy didn't fix itself until there were mass bans. Doesn't mean its impossible to do, just probably difficult.
in regards to taxes:
10% per sale tax still doesn't drain massive amounts of money out of the economy. FFXI is the proof behind this, since it experienced hyperinflation and the AH tax did nothing to correct for it. Possibly a 40% tax on sales would work, or some kind of incentive to sell things -A LOT- more than currently. But right now people don't buy enough for taxes to be a gil sink. I rarely buy anything from the wards, and for high priced items i trade to avoid the tax. Now if they taxed trading as well, thats another story.