Take them out.
They're causing hell on the GW market.
You put them in to stabalize the economy, all they're doing now is inflating everything.
Example: Black dye. I know it's an overused example, but it's a good one. In the betas you could only find a dye trader in Lion's Arch. Upon release they were available in every major city (though, I cannot find the rune trader in the Henge of Denravi

Now, the NPC sells for 20k, and will buy it for no more than -- get this -- 2-3k. People sell their black dye in the trade channel, as opposed to when they could sell it to the vendor for ~4k.
Now, WTF!
And before you hide behind how players do that to each other, buying things low, selling things high... we take opportunities, we don't try to force the prices.
I had 3 silver dye several days ago, I went to sell them to the NPC. He was selling them for 800, but wouldn't give me more than 300 for them. I silently told him to @#$% off and went to put them back in my vault so I could sell them later. But on my way I found someone advertising to buy them. So I sold them to her for 700 each.
See? Your NPCs are doing nothing aside from taking up space the way they are now. Christ, have you looked at the price of superior absorbtions? 80k. 80k! WTF?
And before anyone starts claiming I'm just complaining, that I should just ignore the NPCs, just bite your tongue. You know as well as I do that they're doing more harm than good. Either they should keep up with current prices, or go to hell. Influencing the market is for players and players alone, not some snotheaded vendor AI.
/signed