When it was auctioned in 1999, a document called “Laws of Base Ball” had no known author but obvious significance. Within its pages were fundamental rules like nine men on a side and 90-foot basepaths. But the “Laws” — and two documents inspired by it — sold for only $12,650 in a Sotheby’s auction devoted to books and manuscripts. Now, the buyer is turning into a seller in an online auction by SCP Auctions that will begin on April 6.