It is not legal to use YouTube videos for any commercial purposes and derive any monetary benefits from it. According to YouTube one shall not copy, reproduce, distribute, transmit, broadcast, display, sell, license, or otherwise exploit any content for any other purposes without the prior written consent of YouTube or the respective licensors of the content. So unless you have a written permission from both content creators as well as YouTube, your act of converting and downloading YouTube videos will be considered to be breaking the T&C & policies and potentially the law. Other than that, using YouTube content for commercial purposes can get you into a copyright, since most of the content on YouTube is copyrighted.