Of course he wasn't dead. Only a fool would believe that. It wasn't the book Sarah had been trapped in. It was the metal buckle. Actually, she had been trapped in the seal the whole time. The book allowed her descendent to release her, and then she was transferred to the book. When it was destroyed, so was she.
Ben had thought fast, and done a rapid spell that trapped him in the buckle instead of the pages of the ancient book. He had gathered enough power from Sarah as she was burned, and he patiently waited. He could see everything, and feel everything. The buckle was his body now. Contained within it were his memories, senses, and most importantly, his emotions. He was filled with burning anger, and a drive to destroy Mystery Inc. Except Velma. He had wanted her for his own from the start. She was slightly too young though, so he waited.
After the book was gone, the ashes covered the buckle, hiding him. A burning log wasn't enough to melt the metal, and even if it was melted he wouldn't be destroyed. Matter, as all who have studied science know, cannot be created or destroyed. As long as he was bonded to the steel, he couldn't be killed. He was now immortal. So he waited for nearly fourteen years for Velma to match his age. At that time, he emerged. And the buckle was at the center of his brain.
Anybody who would have been in the puritan reenactment village would have seen a small pile of dirt shudder, then a steel buckle rise in the air. A body would have slowly formed out of thin air, like a phantom becoming real. A ghost became solid, and Benjamin Ravencroft was back in the world of the living.
He smiled, and the whole world suddenly felt a few degrees colder.