Vinyl record grading

I agree, it's about buyer expectations. You want to protect 100% rating.

I don't feel competent to use grades like others, so I came up with two categories:

1. Darn Good: visible but inaudible scuffs

2. Very Darn Good: no visible scuffs

I don't sell anything with even a small scratch or bimp, and I give unconditional returns on everything I sell. 

the covers, I let the photos speak for themselves, with specific notes if needed.

Mono LP's scuffs, slight warpage: may not be audible with your mono cartridge, but a buyer might play it with a stereo cartridge and those scuffs, slight warps could be apparent. Test that with your stereo cartridge, then inform buyer, quiet when played with a mono cartridge.

I had records pulled out from the jacket sounding noisy. For example the original pretty famous and common double-album "Who's afraid of Monk". Had it sealed since long time ago, but once landed needle it started from cracking noise and had it through all of the records. 

You have to clean it with an ultrasonic record cleaner - that might take away some of the ticks/pops.  From my experience, it has to do with how the records were stored.  If the records were stored in a ~ dust free environment, the chances of them sounding good after an ultrasonic cleaning is better. However, if they weren't stored properly, the chances of the ticks/pops going away are slim. I try and return records that have scratches, ticks/pops and impact playback - depending on if they are rated NM or nigher.