Fallout 4 Merchants

Merchants can be found in various locations throughout The Commonwealth in 2287.

"Level 4" merchants
The addition of the settlement system in Fallout 4 allows players to assign any settler to a store, allowing the player's own merchants to be created. Providing a small amount of caps overtime to the Workbench, they allow another avenue of random shipments, junk, ammunition and clothing beyond pre-existing merchants.

Certain NPCs in the Commonwealth however will provide much better equipment and junk than just random settlers, including the sale of unique (legendary) armors. Dubbed as the "Level 4" merchants, they appear to only provide their bonus equipment in Level 3 stores and almost always outperform a random settler in terms of quality and quantity, provided they are assigned to their specialised store.

The most difficult to obtain "Level 4" merchants will only work for a player's settlement if the population of itself and settlements connected to it by supply lines all total to a certain amount of people, with the largest demand being 30 people.

Shipments
Some merchants offer bulk shipments of crafting components in their JUNK category. Shipments are of 25, 50 or 100 units depending on the rarity of the component. Shipments are the fastest way to get rare components for crafting advanced mods. However, it's much cheaper to get common components (Wood, Steel etc.) by scrapping debris around your settlement.

Unlike other merchants who offer shipments, Alexis Combes always has only one shipment available at any given time, but which item is in the shipment changes. The timing or other trigger for this change is currently unknown. One method to cycle her shipment stock manually to get the desired one is to kill her then reload the game.