Yeah, as Tim said the forums like WUS and Timezone may work - there are lots of threads about newer watches so I think the expertise leans to nerwer but there are some threads on vintage, and there are collectors/aficionados on the sites. WUS is more user friendly to use. There's also Chrono24 for watch listings - I think private sellers can list on there too.

The Omega Speedys are nice watches and I waffle between getting one or not...but so far have leaned to a little more niche/rare brands. They're pretty common so that's good and bad but they seem to hold value decently since they have broader market appeal (they spend a lot on consumer marketing) versus a niche/specialized high end brand. There's Rolex's junior brand as well, Tudor, but I'm not sure if the Speedy or Tudor would hold value better. You can also look at US based grey market (genuine) retailers to save a few dollars on a new watch (versus pre-owned).