Is one who does not attend the Eid sermon sinful? And what are the proofs for that?

The Eid sermon is a recommended sunnah (practice), and attendance or listening to it is not obligatory. Therefore, whoever performs the Eid prayer may leave, and there is no harm upon him. This is by the general consensus of scholars. This is supported by the Prophet's (peace be upon him) statement: "Indeed, we deliver sermons. So, whoever wishes to sit for the sermon, let him sit, and whoever wishes to leave, let him leave." Consensus has been نقل (transmitted) regarding the non-obligatory nature of the Eid sermons.