Artisans, including merchants, are a fully playable type of character. They are the instigators of moving goods and currencies around the world. They have the possibility of creating new items, new magical items and entire item classes. An artisan with sufficient training and experimentation may research a completely new item. A specific artisan may become the only person within the game that can create a given item, essentially cornering the market for that item. Meabwhikle a merchant may open a trade route delivering specific goods to a specific market. The rulers of the country in which this occurs may grant sole trading riights to the market. The merchant has a monopoly on that trade route.
For example an 8th level mage, after a close call, decides to give up adventuring and retire. He decides to invest in a level of blacksmithing in order to be able to work with metal. He then becomes proficient in blacksmithing. Realising the money is in magical weapons he picks up a few levels in magic. He now knows how to create simple weapon enchantments and thus can make magical weapons.
The competition in simple weapon enchantments becomes fierce and he starts looking at making more advanced weapon enchantments. He finds he needs to be able to work with finer metals and thus either apprentices to a more skilled blacksmith, or swaps minor enchantment tokens for training, in order to gain the appropriate skill. Likewise he could just buy the enchantments of another mage.
This though leaves him in a bit of a conundrum. He was currently searching for the materials he needed for his weapon enchantments. As he has decided to apprentice to the blacksmith, this leaves him little time to journey out into the wilderness to find the materials he needs.
One of his old adventuring colleagues is a ranger. He cuts a deal with the ranger to supply him with the ingredients he needs in return for the location of a rare herb he needs for the weapon tokens. He believes as the Ranger is a forester that he, the ranger, will not over harvest the herb.
The Ranger finds out that the herb is in great demand by apothecaries as well. So he sell the herbs to an apothecary in return for the apothecary teaching him to make specific potions that will be of assistance to him by adventuring.
A merchant learns of the ranger’s supply of the rare herb and cuts a deal with the ranger to supply not only the rare herb but other herbs, leaves etc that are known to occur in the area. The merchant has learned that there is a magical four legged creature raiding outlying farms. The merchant believes that this creature might be a displacer beast and funds the Ranger to kill the beast and return the carcass to him.
So while an artisan could be a minor subclass of an existing player such as the Ranger or Mage, for the merchant it is his prime class with maybe cleric as a subclass in order to obtain a truth spell or mage for charisma spells.
Over all artisans and merchants provide a complete playable class that glues trade throughout the world. There is no longer the automatic buying of trash articles within the game. Each item must be needed by someone in order for it to have worth. And yes a market could become saturated making an item worth next to nothing.