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. Depositing a small amount of caps over time to the Workbench, they allow another avenue of random shipments, junk, ammunition and clothing beyond pre-existing and wandering 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 or "Specialist 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 specialized 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.

Though the Vault Dweller's Survival Guide for Fallout 4 claims Holt Combes can be recruited as a vendor for settlements, he actually has no special shop assigned to him in his NPC file, nor any settler variables set in place. Oddly Tina De Luca, another Vault 81 dweller that can actually be sent to a settlement, has partial variables set in place to act as a specialty vendor.

Shipments
Some merchants offer bulk shipments of crafting components in their JUNK category. Shipments are of 25, 50, 100, or 200 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. Saving the game before trading with her allows you to load that save for a different shipment, or you can wait/sleep for 48 hours.

Bugs
(possible fix) Remove all charisma boosting clothing, and then put it back on, speak to the merchant, exit, then try to buy from the merchant. Try selling a bullet or two to make sure the price is in your favor.
 * When trading with Trashcan Carla and/or some other merchants, some players have verified a glitch that occasionally occurs which results in the caps transaction not being accounted properly resulting in loss of caps. Then, the bug seems to carry at random. We were trying to pinpoint which character (or event) triggers this issue but so far it does not appears to be NPC/Event related.

(Its a pain) but you can follow, and protect your level 4 merchant to a near by settlement, then once you've assigned them to anything BUT the job they want, open workshop menu and move them to where you want. For some reason they seem to get from settlement to settlement fine for me.(PS4) (also) if you're playing survival, the roads/routes they take are generally the safest way to travel.(boring to follow) but helpful to know.
 * Some Level 4 Merchants won't travel to or stay at your settlement after being assigned there. Solution: after the merchant has been assigned, disable them and any other travelling companions they spawned with, such as accompanying mercenaries ("prid [ref id]", "disable"); fast travel away from the area they spawned in; then walk, don't fast travel, back to that exact same area and a different random encounter should occur, such as wild mongrels or synths; if so, re-enable the merchant ("prid [ref id]", "enable"); fast travel to your settlement; and finally, move the merchant to you ("prid [ref id]", "moveto player"). Now the merchant should stay put.
 * Holt Combes is listed as a lvl 4 merchant on this wiki, but you can never recruit him to one of your settlements for him to become a lvl 4 merchant

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