The American Booksellers Association has sold its headquarters in Tarrytown, N.Y., for $5.75 million, to Diamond Properties of Valhalla, N.Y., a real estate company. The association, which paid $2.8 million in 1991 for the property, will remain as a renter for at least two years and has an option to continue renting after that time. ABA president Mitch Kaplan said the sale price reflected "massive overbuilding and high vacancy rates" for commercial property in Westchester County.