I would like to see my character picture, upon choosing the barter option from the dialog menu, like when you are in the inventory, with your character equipping the items when you click on them, needing to click again to buy it (instead of appearing the do 'you want to buy' option, which should be maintained for not equipable objects). This way you could try different pieces and decide which combination fits better, before buying it.
Alternatively, it could be the shopkeeper and his/her spouse or generic mannequins who equip the clothes. This could be less fitting for this game but could make easier to buy clothes for different gender companions if they are in vanilla game, or once they are added by mods.
What do you think?