Thoughts? It's gotten worse over the years but can't tell if it's just stain of some sort or actual corrosion? Doesn't actually look like rust, and it's only on this section, no where else.

Doesn't look like rust.  Stainless will corrode and get dark spots like that.  But, there is conflicting information on whether the Mariner was ever actually stainless steel and modern versions apparently are not - just coated to look like stainless.  Truth is, all of my guns get used so a minor discoloration like that would barely be noticed here.

All good, moreso just making sure it's not rust. Mainly for resale purposes and wanting to know if there is anything I should do to fix it for future sale purposes, before it gets worse. Contemplating selling it for a different shotgun anyways.