It is '
a used...'
Example
"I want to buy
a used auto, not too old."
NOT "I want to buy
an used auto…."
The rule is
- 'a' is the correct indefinite article when the following word starts with a consonant sound. e.g. a car, a house, a yellow flower, a yew tree, a university (yewniversity)
- 'an' is used when the next word starts with a vowel sound. e.g. an opera, an angry person, an hour (our), an upside-down cake, an umbrella, an utterly useless piece of information, an extremely good answer.
EXPLANATION
The sound of speech doesn't flow easily when two distinct vowel sounds follow each other, so an 'n' is added to the 'a' to prevent this happening.#
Compare, by saying:
a apple / an apple.