Is it permissible to watch and use scientific videos that have been published for free, while their original creator sells them, knowing that the publisher likely obtained them through legitimate means?

If the person who obtained the videos did so through legitimate means, then they are permitted to publish them for free. However, if their publication is unauthorized, then it is not permissible, because intellectual property rights are protected by Islamic law, and publishing them would be considered an infringement upon the owner's right. It is also not permissible for you to download and view them.