used /juːzd/, adjective and past form of use. Spelled u-s-e-d. After the voiced /z/, the ending is the voiced /d/, /juːzd/; never write useed or usd.
The /juːst/ sound belongs only to fixed constructions like used to, not the plain past verb. The spelling stays u-s-e-d either way.
They bought a used car from the dealer. As an adjective before a noun, used means second-hand, owned or driven by someone before.
Please throw out that used teabag. The same written form also describes something that has already been used.
Phonetic trap: the final letter is never silent. After the voiced /z/ in /juːzd/, the ending is a voiced /d/; the voiceless /st/ ending appears only in the used to construction, while the standalone word is /juːzd/. The spelling u-s-e-d covers both pronunciations.