Is it theft to put the mattress in the corridor and sell it privately after cleaning it?

This is a criminal offence. You can call the police. But there must be evidence, such as videos, photos and so on.

According to the amount involved, the court convicted and sentenced him. If it does not constitute a crime, the public security organ shall impose administrative penalties on public security.

The sight of residents piling up sundries in the corridor is not only easy to cause security risks, but also may lead to disputes.