Artisans, including merchants, are a fully playable type of character. They are the instigators of moving goods and currencies around the world. They can create new items, new magical items and entire item classes. An artisan with sufficient training and experimentation may research an entirely new item. A specific artisan may become the only person within the game who can create a given item, essentially cornering the market for that item.
Meanwhile, a merchant may open a trade route delivering specific goods to a particular market. The rulers of the country where this occurs may grant sole trading rights to the market. The merchant then 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 to work with metal. He then becomes proficient in blacksmithing. Realising the money is in magical weapons, he uses his magical knowledge to imbue weapons. 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 apprentice to a more skilled blacksmith or swaps minor enchantment tokens for training to gain the appropriate skill. Likewise, he could buy the enchantments of another mage.
This apprenticeship, though, leaves him with a problem. He was searching for the materials he needed for his weapon enchantments, but as he is now an apprentice to a blacksmith, he has little time to journey out into the wilderness to find them.
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 that the Ranger, as a forester, will not overharvest the herb.
The Ranger finds out that the herb is also in great demand by apothecaries. So he sells 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 also other herbs, leaves, etc. that are known to occur in the area. The merchant has learned that a magical four-legged creature is raiding outlying farms. The merchant believes 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. The merchant could subclass as a cleric to obtain the ability to cast truth spells or as a mage for charisma spells.
Overall, 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.