I always like to assume that every city if populated by NPC citizens and guards and peasants and businesses and everything that the given city would be expected to have. That means you can expect Witchaven to be full of fishermen and fish-related services, and you can expect Varrock, market capitol of the world with the Grand Exchange in their borders, to be full of rich trade diversity for most any kind of product you want to buy.
The only difference I would address when talking about very rare products, like rune armor, is that the affordability of prices in the shops that have those rare things for sale. To us as players, paying 60k for some rune armor is like sneezing. On top of that, the prices on those items were put up based on Runescape's economy in, like, 2003 - when 100k for full rune was hard to earn and wearing a dragon med helm was a symbol of great experience and wealth.
What I'm saying is, it's completely possible that a party of adventurers from the Champion's Guild could have gone out and slain a dragon and found some rune armor in the dragon hoard and are trying to sell it so they can split the loot. However, if you agree with my ideas about the realistic value of money and items in roleplay, it would be financially devastating for even a person of great wealth to purchase full rune armor. (That is, in my system, even a person from a family of great wealth would have to live on savings and spend none of his income for 100 years before he earned the approximate suggested price of an entire suit of rune armor.) So, while walking into the champion's guild to purchase a suit of rune armor may be very feasible as an explanation for how you got it, you do still have to explain where your character got more money than a small, flourishing town's entire treasury.