Now on the bargain shelf is "The Anchorage," summer home of the late Nelson A. Rockefeller in Seal Harbor, Maine. The secluded residence, which originally was priced at $1 million, was withdrawn from the market at the time of his death last January. It has now been put back on the market at just $650,000.

The broker for the Rockefeller estate, as for many large, luxurious properties around the world, is Sotheby Parke Bernet International Realty Corp., a division of the New York-based art auction house. Its president is Charles H. Seilheimer Jr., formerly a real estate broker in the Virginia hunt country. Recently he announced that his company had sold more than $5.5 million worth of properties in the first half of its current fiscal year and has contracts on $2 million more. About half of the properties have been sold to foreigners.

Its most recent sale was "The Meadow" in Doswell, birthplace and training ground of Secretariat, the Triple Crown winner. A group of Virginia investors bought the breeding farm for more than $2.5 million. Among those places now being offered by Sotheby are "Blenheim," a 257 acre estate belonging to the grandson of lyricist Oscar Hammerstein II; the $1.9 million "Beauregard," site of the Battle of Brandy Station; "Menlough," a 55 acre estate in Warrenton; and "Nanzatico," a manor house on the Rappahannock that is said to be the finest 18th century fram structure in Virginia after Mt. Vernon. CAPTION: Picture, Nanzatico, one of the best frame historical houses.