I realized that my heavily armed merchants were selling their own weapons. Does anyone know of a simple way to prevent this? Currently I am removing the weapons and ammo just before the player opens the barter menu, then place them back after. This seems like a bit of a kludge. Isn't there an easier way?

Set the weapon to not playable, and set its ammo type to one of the robot ammos.

Thanks, I'll give that a try. I assume the NPC is still able to use the weapon?

Yup, setting an item to unplayable just makes it not appear in inventory lists. No dropping, selling, trading.

Ah, so to make a weapon unplayable, you have to create a new weapon. So this will not work with stock weapons. It seems a little bit more cumbersome to create a whole new weapon class, than to simply remove and the return a weapon to the NPC. Not sure which way to go.

The issue with the remove and return is that you can actually see it happen for larger weapons. Strange that there wouldn't be a more elegant way to do this. Like just setting the weapon to being owned by the NPC.

The simplest way is to pick a vanilla weapon, proceed as if you were going to change the name. When prompted - 'create a new object'. Set it as non playable. Edit the ammo to use the appropriate robot ammo. Done.