You know the old expression, "Grit happens!" right? I mean, you've seen it on bumper stickers for years now, right?
The atmosphere is filled with very small particles of dust and grit and pollen and junk; it finds its way inside of lenses that are used. The older a lens, the more junk it might tend to have inside of it.
I have a 1982 105mm f/2.5 AiS Nikkor lens I bought brand new, and I have a very early 2000's model, identical, which I bought in mint condition used. The 27-year-old model that I carried and used every day for years has a lot of "gunk" inside of it; the newer model is almost pristine. But you know what? The images coming out of either lens look basically identical.
MOST older, used lenses have some stuff inside of them....it's not that damaging to the images under most conditions, If you go out to your car and look at the paint finish really,really closely, you'll see hundreds of thousands of minute scratches. Take a magnifying glass, and LOOK--you'll be aghast at the damage that your paintjob has. Molehill does not equal mountain.