You can soak the rusty areas with Kroil or ClP Breakfree and let soak a couple of days to penetrate and soften the rust.
Then apply more and use a brass "toothbrush" to scrub the areas.
This will remove the rust without risking damage to the rest of the bluing.
After the rust if off, apply CLP Breakfree as a preservative to prevent rust, and to continue to "work" any remaining rust.
What will be left is missing blue and tiny pitting where the rust was.
About the only way to correct that is to return the gun to Colt for a refinish to like new condition.
I don't recommend using any cold blues to try to hide the damage.
Cold blues contain acid that can actually cause more rust, it turns brown with time, and you can rub it off with your fingers with a couple of swipes.
Worse, cold blues leave permanent blotches in the original blue around where the cold blue was used.
Possibly the roughness is just a dirty action and cylinder. You can clean everything up and that might correct it, or it could be a mechanical problem.
Whatever, don't try to disassemble the cylinder unless you know how it's done, or you risk doing really serious damage.
So, sight unseen it could be a good buy or a problem that would cost to get repaired by Colt.
The good point is, of you can get it cheap enough you could send it in to Colt for restoration to like new condition and still come out ahead.