I skimmed through the wiki on Roberts and Seth. My personal experience as a writer of fiction, usually about science fiction, fantasy, and the paranormal, when I went on to do something real, yet largely unbelievable or disputed (though not paranormal), I got a lot of flak for my accomplishments because of that. That's what leads me to be somewhat skeptical of the story, Roberts being a fiction writer and largely dealing with the paranormal and psychic phenomena, but I wouldn't doubt it outright if it were true.
I feel that someday science will finally answer the question of ghosts and such, but until then the people who believe will believe and the people who don't won't. I still stand by what I said about the Ouija board being nothing but a board game. I doubt anyone would mass produce and market something everyone actually believed could potentially open a doorway to hell. I don't deny that paranormal events occur around the users of the boards, but all the claims of it being the board's doing is superstition in my mind.
When people perform the ritual of cleansing the board and praying and whatever else, they open themselves spiritually, using the board simply as a conduit much like how Christians use a cross as a conduit to pray to God. It's not so much the item used, but the essence of the ritual that allows these events. Proof being (if the Seth story is true) Roberts eventually being able to communicate with Seth and Seth (supposedly) possessing her without the use of the Ouija board. As she more constantly allowed herself to stay in the spiritual mindset, the circumstances allowing the events were more constant, allowing them to happen more easily.
