Would I be sinful if I established a website for marriage according to Sharia conditions, and then one of the users violated these conditions?

Such websites, if governed by Islamic legal controls, are a great door to good and an example of cooperation in righteousness and piety. The mentioned controls are good and must be diligently applied. If effort is made in governance and someone manipulates without negligence, there is no blame, for Allah Almighty says: "So fear Allah as much as you are able."