A small failed Upper East Side co-op building where all 25 units are now market rate was sold through Auction.com for more than $15 million.

The walk-up’s auction generated 257 requests for information and eight active bidders with the winner a Brooklyn-based operator.

Because of the ownership structure, the transfer was made via the sale of 100 percent of the cooperative shares. “He has infinite possibilities,” said Auction.com exec Pierre Bonan, as the buyer can upgrade apartments and continue to run them as market-rate rentals, convert to a condo or effectuate the co-op. There are also enough air rights to create a nice penthouse on the top.

The property was purchased from an estate in the 1970s for a mere $70,000, and various family members moved in before an attempt at a co-op did not generate enough sales. More recently, the building had been offered for sale “to the usual suspects” through another brokerage firm before the seller turned to Scott Bloom and Kent Covington of Bloom Real Estate Group.

“The Auction.com process allowed us to cast a wider net than we could otherwise — and yielded a first-time Manhattan buyer,” Bloom explained.