You should explain that the Salve heals 400 hp over 10 seconds and that clarity is 100 mana over 30, and to actually make sure the hero is low on hp/mana to demonstrate how well salve/clairty work. You could do this by getting attacked by Razor as you enter the field but then he flees or some creep etc to reduce the heros health. You could also demonstrate how an attack removes the heal by having an 1 hp range creep hit you that survived the initial attack against you, then the merchant could sell you another or something.