Cornelia Guest has put her family’s historic estate in Old Westbury, LI, on the market. The 1980s “Deb of the Decade” — daughter of polo legend Winston Guest and socialite and New York Post gardening columnist CZ Guest — wants to sell her 11-bedroom, 1924 ancestral estate for up to $20 million. She was overheard telling guests at Martha Stewart and Harry Slatkin‘s birthday party last weekend, “I want to move to Montauk.” A rep for Guest confirmed, “The house is back on the market.”