Weapons trader has no stock
I've built a level 2 weapons shop in a settlement, assigned a settler and when I try to use it, they have no stock, nor do I have any inventory (apparently)
My general-trader doesn't have this issue, any tips?
There are multiple reasons for a shop to not have any stock:
You just built the shop and assigned someone to it. The shop doesn't have a stock yet, because the trader hasn't had the time to stock up on goods. When this happens, you should still be able to sell stuff, but there won't be anything to buy. The shop should be working normally the next day.
The shop isn't open yet. It's either past 8 PM, or before 8 AM. While the shop is closed, you can trade items with your shopkeeper the same way you trade items with any other settler or your companions. However, it's possible for an empty shopping window to appear at 8 AM/PM, right when the NPC is switching between settler mode and shopkeeper mode. Simply leave the shopping window, wait a few in-game minutes (or an hour to be sure), and everything should be working fine.
You just assigned a new shopkeeper to an already running shop. It's similar to when you just built a new shop, but with the difference that you shouldn't need to wait an entire day before you can use the shop. Waiting a few in-game minutes (or an hour) is enough.
The shopkeeper is too far away from his shop and has bugged out because of it. This can happen when the game's pathfinding algorithm failed to guide the NPC towards his shop. In this case, you have to guide him there yourself step by step. Simply force him to move somewhere; make sure you wait until he gets there, before you move him to the next point. Keep issuing move commands until he's reached his store, and assign him to it again. Make sure you keep an eye on that particular shopkeeper in the future, though. Some NPCs simply don't like working as shopkeepers (probably just a bug) and will rarely tend their shop. If you see that, you might want to choose a different settler for the job.
The shopkeeper bugged out for any unknown reason. This has happened to me a few times, but luckily not very often. You can usually fix this by leaving the shopping window and trying to barter again. If that doesn't fix it, try again another day (or save and reload). If it still doesn't work, try to assign a different settler to the shop. Make sure to assign the shopkeeper to a different job before you select a new shopkeeper, just in case.