A three-story Park Avenue town house never publicly listed for sale has changed hands for $13 million, making it the most expensive transaction of the week, according to city records. The sale of the building, at 1108 Park, between 89th and 90th Streets in Carnegie Hill, comes just two weeks after the sale of its neighbor, 1110 Park Avenue, for $16.5 million. Both were bought anonymously by the same limited liability corporation, but papers filed with the city indicate that the buyer was the residential developer Toll Brothers of Horsham, Pa. The company declined to comment. The 25-foot-wide town houses, dating to 1856, are among the oldest buildings on the avenue. 1108 was sold by the estate of Joseph Bogen, a longtime resident who died last year. 1110 was once owned by the Abstract painter and philanthropist Judith Rothschild. Both houses predate the construction of the large luxury apartment buildings that started sprouting up at the turn of the last century and now dominate the avenue. Although Toll Brothers has not officially acknowledged buying the properties, and no plans for redevelopment have been made public, some local residents said they were bracing for a fight. The two houses fall just outside the Carnegie Hill Historic District; neither is protected by landmark status, and they come with substantial air rights. In other words: an ideal location for a developer with big ambitions.