I can imagine several ways of being able to achieve this, for example, a Roulette wheel - you place the rune or spell in the empty slot on the wheel pay a small fee and spin the wheel. you come away with what ever the wheel lands on. An action house you put in spells or runes and people bid on them say for 5 days. How about having a magic tankard in the wayfarer's tavern, drop in the rune or spell and tip out another changed by magic beer. Also just you add the feature to the trader section and ask for what you want instead. I like the Roulette idea, you could have diamonds, coins, runes and spells on the wheel and as with the trader the price goes up with each subsequent spin. But you would have to also build a casino. So it can be done just use the imagination.
LordT