Shipment caravans travel in PvE all over Tyria delivering special items to the specific special shops/merchants.
If the caravans make it there, those goods become available at those merchants.
Shipped items need to be worthwhile—exotics, ascended armour and weapons.
Global realtime merchant stock supply,—when the stock is bought by players, its gone until the next shipment.
Dynamic event raids on both the caravans and the merchant shops by PvE mobs.
If raids succeed, mobs get those items to use—exotic, ascended armour weapons! (You would see the bandits wearing and wielding that gear).
Dynamic events to get those items back from the raiders.