The rule is that you use “an” before a vowel, but that’s understood to mean before a vowel SOUND. " used " begins with a consonant SOUND, a “y”. So phonetically, you’re saying, “YOO-sed.” Therefore, you must use “A used .” " An used " violates the rule.
could’ve used ukelele, uniform, or youth for better examples
a ukelele - She plays a ukelele.
a uniform - He wears a uniform to school.
a youth - She is only a youth.