"It's time to move on," said Alter, who is now involved in beerjobber.com, a craft-beer marketing company.
The L-shaped parcel encompasses 70 acres - equivalent to an area encompassing 10th Street to Broad Street and Arch to Locust Streets - and for the claustrophobic, the agent says 10 additional acres are available.
The property has a county estimated market value of $24.2 million, with a $355,866 annual tax bill.
Wharton School real estate expert Susan Wachter said that residential sales of such magnitude aren't that unusual - in Singapore, from which she just returned. "But for Philadelphia," she said, "this would be quite a sale."
"It is the most prominent property in the region," said Reggie Hunt, a realtor with Long & Foster in Wayne, which is affiliated with Christie's International Inc. of New York. The listing agent is Janet Rubino.
The property is being marketed nationally and internationally, said Hunt, but "we have had some local people express interest."
She added that it might be attractive to developers. "This could be subdivided down the road," she said.
In addition to the residence and play house, the property includes two stone farmhouses, just in case guests show up.
The main house, designed by Viñoly, a friend of Alter's, is built of stone, wood, glass, and Venetian glazed plaster.
It was erected when Advanta was in its heyday. The credit-card company, founded by Alter's father, filed for bankruptcy in November 2009 after its fortunes declined spectacularly.
Alter hired expensive executives, including an Icelandic novelist/video-game designer, some of whom ended up suing him. Along the way he made unwise investments.
Alter and his ex-wife, Gisela, have four children, and Alter said that since the divorce the house now is occupied only by him and their two sons "periodically."
Said Alter, "I don't need 70 acres anymore."
Inquirer staff writers Joseph N. DiStefano and John Duchneskie contributed to this article.