On the sidebar on p. 225 is where it says "in some exotic locales, residuum is traded as currency." I'm thinking this is only for rare and usually planar locales, where everyone, their mother, and their seventeen siblings can cast Disenchant and Enchant Magic Item—in other words, the kind of locale you almost never find in a "Points of Light" setting.
On p. 300 is where they say "you can't usually buy it on the open market." That's a more general statement; you can't trade r in most terrestrial towns and cities because of the rarity of people who have or want it.