Merchants can be purchased at your house's hookpoints (or at cross-realm NPC merchants in central marketplaces on Gaheris and Mordred). Shift + right click on an interior wall to bring up your House Internal Options Menu and click on Hookpoints to display them. Hookpoints will appear as concentric circles on the floor. Right click on a hookpoint to bring up the hookpoint merchant menu. Drop the merchant or NPC on the hookpoint to place it.