Learn Italian: Al mercato, si è venduto ogni cesto di cuoio. - At the market, people sold every leather basket.

The verb vendere usually takes avere in compound tenses, like ho venduto (I sold). However, when you use the impersonal or passive si, the auxiliary verb always shifts to essere. That is why it becomes si è venduto.

Since the auxiliary is essere, what decides the ending of the past participle venduto?

Because we are using essere, the past participle must agree with the grammatical subject. In this passive si construction, the subject is the thing being sold: ogni cesto (every basket). Since cesto is masculine singular, venduto ends in -o.

What would happen to si è venduto if they sold multiple baskets instead of a singular one?

If the subject were plural, like i cesti (the baskets), both the auxiliary verb and the past participle would change to match it. The sentence would become si sono venduti i cesti.

Can I use the preposition in instead of di for the material cuoio?