What are these spots inside the rear element of my Nikkor 28mm f/2.8 AI-S lens?

My problem looks like it's between the glass elements.

When the cement between lens elements fails, the problem is called Balsam separation (or just separation). Canada Balsam was used as an optical cement in very old lenses, but has been long been replaced with other materials in modern lenses.

The problem is not "Schneideritis". Schneideritis looks like white dots in a black area around the edge of the glass. The problem is caused by the flaking of black paint that was used to prevent stray reflections inside the lens.

Since the defect is along the edge of the lens, it may not be visible in images taken when the lens, especially when the aperture is stopped down. However, the problem may worsen over time. The lens is definitely not mint.

Some Japanese sellers use "Mint" in listing titles with horrendous defects in the description (scratches, fungus, dents, separation, etc). So it's important to read listings carefully.

Consider contacting the seller for return. If you purchased through a site that requires sellers to pay for return shipping for defective items, make sure the seller abides by those policies. Some sellers will refuse to pay for return shipping to reduce their costs and discourage returns. If that happens, you can ask the listing site to mediate.