And just like that, yet another one of my addons is now obsolete. With the release of 6.2 Build 20201 (1 July), the issue this addon addressed has been corrected in Blizzard's own code.
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Patch 6.2 introduced a bug when attempting to purchase currency (Elder Charms of Good Fortune, Mogu Runes of Fate, etc) from a vendor. This addon replaces a Blizzard function with one nearly identical but has one extra condition added to avoid this bug.
The exact change is as follows:
Replace MerchantFrame.lua line 581
Code:
if(itemButton.link) then
with
Code:
if(itemButton.link and not strfind(itemButton.link, "currency")) then