Does the questioner bear the sin of establishing a website for movies, and not for a company, after the other party swore that he wanted to establish a website for the company?

You are not accountable for anything as long as you investigated and inquired, and the seller swore to you that he did not intend anything unlawful. In this case, the prevalence of probability takes the place of certainty. We have previously clarified that it is not permissible to assist a Muslim in a sin. Therefore, whoever knows that what he is assisting with will be used for evil is complicit in the sin.