Ben Stiller and his wife, Christine Taylor, are selling their Upper West Side duplex co-op for $10 million. Their five-bedroom, 5 1/2-bath home on Riverside Drive is 4,000 square feet. “They love real estate and enjoy moving every now and then,” a source explained. The posh pad’s already gotten interest from finance types with families looking for an Upper West Side laid-back luxury lifestyle, we’re told. Prudential Douglas Elliman broker Bruce Ehrmann declined to comment.