There are many watches online that are listed for sale for a lot cheaper than what others with the same ref. number go for, but there's a problem, the case back is rusty-oxidated. The question is: would you ever consider a watch with rust? Maybe with the intention of polishing it? Or you would stay away from rust no matter what?

 

For me it would depend on the watch and extent of the rust. If it was me, I would post pictures here on this forum and ask opinions if it's worth it. I wouldn't say I would completely stay away from rust, as I am sure there are certain watches or particular watches for the right price, it could be overlooked.

 

To be honest, I've never seen a rusty watch case or case back.

 

http://www.chrono24.it/omega/seamas...d-winding-mens-watch--bf312088--id7113369.htm
This for example

 

Are you referring to the stuff on the outer edge of the caseback?

 

For me, there is no rust on the watch you have posted. The only thing I can see is the gasket that has melt with time. The kind of black/brown durt is the residue of this gasket.

 

As @jakeh417 said it would depend on the watch if it was worth the gamble
Here is an example of what can be achieved by very skilled watch restorers such as Simon Freese
http://speedmaster101.com/2017/08/
Scroll down to A restoration of a 145.012 movement:

 

I think the OP was asking about rust on cases, not flooded movements.

 

Exactly, maybe the example I provided wasn't the best, here's another one, http://www.chrono24.it/omega/seamas...-watch-2790-head-only-bf313231--id7095996.htm

 

Melted gasket material is black. Any brown you see there has the potential to be rust. Rust on the case back in this location, or in the groove where the case back seal sits is not uncommon. My bet is that if you scraped those spots, you will find pitting under the rust, like this:





Cheers, Al