Greece holds a 55% stake in the airport and 10 investment schemes submitted expressions of interest for the 30% stake on sale in October.
Germany-based AviAlliance GmbH, owned by Canadian pension investor PSP Investments, holds a 40% stake in the airport and Greece's Copelouzos the rest.
"The first evaluation shows that the bids are good," the agency's chief Aris Xenofos told reporters. "We believe that a big part (of them) will move ahead."
After that stage, the agency is expected to invite shortlisted investors to submit binding bids by April 2020, said a report published by the country's EU lenders last month.
The sale is crucial for Greece, which emerged from international bailouts in 2018. The country is aiming for privatisation receipts of 2.4 billion euro in 2020, a goal which is "feasible", Xenofos said.
Greece has raised 1.2 billion euros from state asset sales this year. In total, it has received 7 billion euros from privatisations since 2010.
The conservative government that came to power in July has promised its lenders, who monitor its progress, to speed up privatisations to attract investment and stimulate growth after a decade-long debt crisis.
The state is also considering a sale of its 35.5% stake in the country's biggest oil refiner Hellenic Petroleum via the Athens Stock Exchange.
Its previous effort to sell a 50.1% in Hellenic jointly with the refiner's biggest shareholder, Paneuropean Oil and Industrial Holding, stalled in April as no binding bids were submitted.
Xenofos said the sale still hinged on market conditions.
"We have our antennas out," he said. "We are on guard but it's the market which will give us the signal on the interest which may be expressed or on the price."
(Reporting by Angeliki Koutantou; Editing by Renee Maltezou and Mark Potter)