Merchant

Merchants were citizens who sold various items in the many cities throughout Italy.

High Middle Ages
The role of merchants in Altaïr's time period, was that it could be used as a handy escape option, when diving through their stands, to avoid running through a crowd of citizens.

Missions
In the PC version, there is also a secondary mission called Merchant Stand Destruction, in which you must destroy a set number of merchant stands within a time limit. The explanation for this is that the targeted merchants are selling products that are causing illnesses. In the Xbox 360 version, there is an Achievement for diving through twenty of them.

15th Century
Merchants became a vital part of Ezio's progression. Aside from some of their stalls providing escape routes, he could now purchase various items from them.
 * Art merchants: Art merchants sold paintings, which increased Monteriggioni's value, and treasure maps, which specify where treasure chests could be found.
 * Blacksmiths: Ezio could buy certain pieces of armor and weaponry from blacksmiths. They also sold various forms of ammunition (such as smoke bombs), and repaired broken armor.


 * Doctors: Doctors could heal Ezio's wounds for a small fee. They also sold medicine vials and poison vials (for the poison blade).
 * Fast travel booths: Fast travel booths allowed easy travel to different locations throughout Italy for a sum, and acted similarily to fast forwarding a memory in the Animus 1.28.


 * Tailors: Tailors sold pouch upgrades to increase the ammunition carrying capacity. They also allowed Ezio to customize most of his armor by dyeing it with different colors. This had no real function, however, save for the Perfect Harmony Achievement.

16th Century
Merchants play an even more important role than in the 15th century. Once in Rome, Ezio found out that the Borgia had ruthlessly commandeered the city; under the system of Borgia towers placed around the city, merchant shops had been forced to close and Rome had fallen into disrepair. Similar to the renovation of Monteriggioni, Ezio had the ability to revitalize the city of Rome over the course of time. After killing a region's Borgia Captain and burning his tower, Ezio could begin to rebuild that region's buildings, including ancient ruins, underground tunnels, and merchant shops. These merchants include: New in Rome was the ability to sell items. In a similar manner to selling items in Project Legacy, virtually any item that could be stolen or looted, could be sold for a portion of its value. Of even greater importance were the new "shop quest" side missions. These missions involved trading a certain number of specified rare objects to merchants located on Tiber Island; in return, the merchant would offer an item for sale that was normally unavailable to the player. These items include:
 * Art merchants: Art merchants still sold paintings and treasure maps, but the selection of treasure maps had expanded to include Borgia flags and also feather locations.
 * Banks: Banks performed the same role that Claudia did in during the renovation of Monteriggioni: keeping a record of restoration efforts, and keeping a bank account open for Ezio's income.
 * Blacksmiths: Blacksmiths sold armor, weaponry, and ammunition as well as repair broken armor, much like in the 15th century. However, the crossbow was now available, allowing for silent, distant kills.
 * Doctors: Just like before, doctors could heal Ezio's wounds or sold him vials of medicine or poison.
 * Tailors: Also like before, tailors sold pouch upgrades to increase the amount of ammunition available at one time, and could dye his clothes in different colors.
 * Vatican treasure map - art merchant
 * Seusenhofner armor pieces - blacksmith
 * Fast-acting poison - doctor
 * Large pouch for crossbow bolts - tailor

Trivia

 * Merchant stands in both games do not sell any food or drink.
 * Merchant stands can only be broken by throwing someone into them, killing them (and the resident merchant) instantly. Vigilantes and scholars do not die from being thrown into a merchant stand, as they cannot be killed.
 * The merchant stands that can be traded with cannot be broken.
 * When you climb a building with a merchant, in Assassin's Creed II or Assassin's Creed: Brotherhood, the counter acts as a wall.