These merchants can be found in the large cities: London, Paris and Beijing. They will offer you components for crafting - the smallest assortment in London, the largest in Beijing. Their prices aren't low but they will help you safe a lot of time and some of the "grind". So go and check them out before getting weary from collecting hundreds of materials.
Material merchants will also ask you to do daily, weekly and monthly quests. The daily quests are called "Replenishing the Stock" and will ask you to collect either: 10 Marble & 10 Withered Nightshade, or 6 Pure Crystal & 8 Flushed Clover, or 18 Purple Gem & 8 Green Copper. You can get these quests from any material merchant, can give the collected materials to any material merchant you meet, and you will get good money as a reward - nothing else though.
You can get weekly quests only on Sunday and also only after you have already completed at least four daily quests in the week before. You will be given 10 items (apple seeds, cherry seeds, oriental yam seeds or fruit baskets) and will have to give one of them each to 10 different players. You will get nearly twice as much money like for daily quests.
You can get monthly quests only from 25th to 27th of a month, and also only after completing at least two weekly quests the month before the 25th day. There are 6 different monthly quests and they will earn you big money.