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I don't take CDR runes for AP Ezreal, since he has Mystic Shot, and I always build Athene's on him, which gives 20% CDR. I focus more on burst damage, since you'll need to build up AP for Trueshot Barrage.
Like the others said, it's also advisable to get Lich Bane on him, since his Mystic Shot procs its passive, which will be your main poking tool. After those, it's all up to you what to build, depending on match-ups and situations.