Once you get to Lion's Arch, you are able to sell most everything at its best price (via D1 trade). So don't worry about stuff being more expensive in later areas.
I would sell all the common materials I don't need for armor later on to the merchant, or spend some money and craft them into rare materials to sell to players. Rare materials, depending on the material, can be sold to players for a small profit margin. If you happen to know someone who specifically needs certain materials, keep them as you find them and you can sell them to the person for more than to the merchant.
