As it stands when you examine an item that you are unable to buy (yet) because you haven't acquired a certain Achievement it will show you in red text on the tool-tip what Achievement you need to attain before you can purchase that item.
However, once you've attained that achievement this text disappears so the next time you view that item you can't easily tell what Achievement you earned that now allows you to buy it (this information could be important to have if even just for your own edification on how you earned the right to buy that item - or inform someone who's not at the vendor).
So I'd like to suggest that the text remain, but turned from Red to White (or green).
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PC/NA