Is it permissible for me to copy articles from other websites, mentioning the source, given that I do not know whether they permit copying or not? And do I commit a sin if these websites prohibit copying? And is it permissible for me to profit from my website by uploading products or publishing advertisements if I copy articles from websites that permit it, while mentioning the source?

Copyrights are protected for their owners and may not be infringed upon. The preservation of rights can be known by explicit mention, or by custom and circumstantial evidence. نقل المقالات المنشورة في الإنترنت وعزوها لمصدرها لا يعد انتهاكاً لحقوق النشر. If the transferred content on your site is with the permission of its owners, then there is no objection to profiting from the site by uploading products or publishing permissible advertisements.