Maxperson
Morkus from Orkus
But...but...if it's random and occasional, then you don't need to a price list that you build yourself. You just need that occasional price.
Let's say you somehow randomly determine that a shady merchant has a Figurine of Wondrous Power: Olivine Platypus for sale. How much is he asking? Well, you know your heroes are all toting around about 10k gold, so you make up a price: 25k gold. (I spent a solid 0.75 seconds making that up.)
Is that too much or too little? And how could you possibly tell the difference? I guess if they won't pay the 25k then it's too much. If they eagerly buy it, you probably underpriced it. But so what? But what does the "right" price really mean from game balance perspective?
The pricing is also going to vary considerably. The +1 greatsword being sold by a court dandy that he just inherited from his grandfather Sir Iwasa Selfmade Badass is probably going to be sold cheaper than the +1 greatsword and NPC greatsword wielding NPC fighter is going to sell it. The former couldn't care less about large weapons and probably wants some coin to throw parties, and the latter is going to be using it to save his life against monsters and other enemies.