yes Sir I'm an animal, please don't tell anyone!
when i buy used discs some of them has greasy fingerprints on discsurface from previous owner
one disc had dried residue from fingerprints touching somekind of gunk, i have to clean that disc b4 i play it

it looks like dried elephant seemen or other nasty gunk that PO had on his hand and fingers, this gunk dried on disc surface creating hard smudge I have to sanatize the disc.
buying used stuff is risky biniz if stuff is contaminated with germs!
luckily i wash my hands everytime i have touched a used disc

Germs are the least of your problems.
What about bugs, or even bed bugs, they could be in the sleeve and escape then you have bed bugs.........

What I were talking about is just some really light film from the plastic bag the disc is stored in. I've really only seen it from the plastic bags found mostly in japanese releases. Those bags many people hate, and nicknamed "elephant condoms" because of the shape and size. You'd probably not notice it at first glance either, the disc would appear to be spotless. I've even seen this on brand new sealed discs too. Not sure what it is, but light wiping over the disc gets rid of it.

I rarely see discs with actual smudges on them. Only discs I've seen that on were some movies obviously kids have handled. Besides that it is not uncommon with scratches on the disc surface, but it usually doesn't affect the playback.

Scratches and fingerprints are just something one has to live with when it comes to second hand discs. Most people really couldn't care less how it is handled. I've seen way too many times where people have attempted to clean discs, and scratched them more or create a chemical reaction that ruined it. Stickers on covers is also a common thing people tend to mess up and make the situation even worse. If one is unsure if the cleaning method could cause more damage, just leave it as is.

If you do want to remove stickers though, it is actually easy to do with a simple product called lighter fluid (the type sold in cans to be used in Zippo cigarette lighters). I start by saturating the sticker with lighter fluid and leave it for a couple of minutes to do its magic. Then I take a small piece of paper and start sliding it underneath the sticker. If any resistance is felt during this process give it another squirt of lighter fluid, wait, and continue until the sticker slides right off onto your piece of paper. If there any sticky residue left behind simply wipe it away with a paper towel moistened in lighter fluid. Once the lighter fluid has fully dried there is not a single mark or stain left from having used it...works like a charm everytime!

Meanwhile in sticker removal land it turns out that lighter fluid works better than "sticker remover" from the hardware store. I might start smoking too, just to use a cool zippo..