Is stipulating the absence of illicit transactions on the website sufficient to absolve one's responsibility, and is the website owner considered an assistant to sin if one of the users violates this condition, and is his presence required for the sale, given that he is an intermediary?

If the questioner's work on his site is limited to mediation without interfering in how the sale contract is concluded, then there is no harm in creating the site, provided that conditions are set to prevent prohibited transactions. If a user violates these conditions without the questioner's knowledge or intervention, then the sin is upon himself; as Allah Almighty says: "And whoever earns sin, he earns it only against himself" and "O you who have believed, upon you is [the responsibility for] yourselves. Those who have erred will not harm you when you have been guided" and "You are not charged with responsibility for more than yourself." The questioner, as a mediator, is not required to be present at the time of sale; his role is limited to connecting the seller and the buyer.