is the item supposed to respawn? or is it a special one-time deal for the sale?
if it's a one time sale, i would use dialogue with generic NPC responses (you can find a lot of them under "shared") if you dont have the means to make new vocals. scripting stuff like this inside quests is probably the easiest way to achieve the effect you want
if it's a respawning item, i would still make a quest, but have the sellable item inside the merchant's inventory, and the display weapon as an alias. you would still need to script in the quest to remove the display every time the weapon is removed from the merchant's container