I have some questions on this one. According to the Omega Vintage Database, Ref. 166.(0)003 should have a non chronometer rated cal. 562 or 565 movement.
https://www.omegawatches.com/en-us/watch-omega-seamaster-omega-cd-166-0003
However, this example has a chronometer rated movement and appropriate dial. Generally, when Omega produced a chronometer version of an automatic Seamaster model, they assigned a different case reference number, usually starting with “168”. We do not see that here.
My guess is either the caseback was replaced at some point, or the movement and dial were transplanted from another watch. I also think the rotor was replaced with one from a later movement. The parts of many Omega movements from this time period are interchangeable. We sometimes call this a “Frankenwatch”. It will never be a true collectible watch.
All that being said, it’s likely worth every penny of the $400 being asked. That’s a low price for an entry level Omega, and this one has one of the best movements they ever made. I just would not spend huge money on a complete service and would not treat this as an everyday watch.
It’s certainly a nice looking example.
Hope this helps,
gatorcpa