Go into main towns or perhaps even your guild hall if it has all the services. Hold down the Alt key and wander around looking for green NPCs that say "Material Trader."
Here's a wiki link explaining what those guys do ...
http://wiki.guildwars.com/wiki/Material_trader
Quote:
Material traders are NPCs that trade in common crafting materials. These NPCs are recognized by the "[Material Trader]" tag in their names. Unlike regular merchants, a material trader's stock and price is dependent on the supply and demand of the player base.
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Talk to that NPC and they'll show you stock. You can buy common materials, like dust and fiber, in piles of 10 each for the price he quotes. If you have 10 dust or fiber, you can offer to sell it to him. For most common materials, the price he'll buy your 10 for is usually 100g less than the price he'll sell you 10 for, but it never drops below 30g.
So to find out the minimum he'd sell you 250 dust for,
( current quote for selling 10 dust ) and multiply it by 25.
You multiply by 25 because he's quoting a price for 10 and you're trying to find the price for 250.
To find out the minimum he'd buy another player's 250 dust for ....
( take the greater of 30g or {current quote for selling 10 dust - 100g} ) and multiply it by 25.
Now you know the current market price range for stacks of dust. Most players trading in stacks of materials to each other just tend to average the trader's buy and sell prices.
Cheers & GL!
EDIT - be careful this particular week about dust. The traveler is collecting glass vials, which can be crafted using dust. So that may inflate the prices during this week.