The art is a page from the story The Phantom Fleet, first published in Eagle 50 (volume 9) in 1950.
In the story, radio and television transmissions are being disrupted throughout the Solar System by an unknown force. Spaceships are disappearing. After a rocket ship carrying Sir Hubert Guest goes missing, Dan and crew set out to discover what has happened to it.
They find a fleet of huge alien craft containing peaceful aquatic creatures called Cosmobes who are fleeing from another aquatic race, the warlike Pescods, whose "Crimson Death" weapon destroys all metals on contact.
Attempts to destroy the Pescods' pursuing fleet fail, and they dive into the Earth's ocean and begin to build themselves a city. Fortunately for Earth the Pescods' settlement is on top of the submerged volcano Krakatoa, and when it erupts due to seismic disturbances from the pesky Pescods, they are all destroyed.
|A panel from the offered Dan Dare page|
Scott previously bought and sold original comic art since the early 1980s, but "gave it up full time in 1995 to work in comics publishing."
Could be worth keeping an eye on.
• View The Phantom Fleet art here on eBay
• Dan Dare comics checklist (GoogleDoc, corrections, additional information welcome)