You can be lucky with a grade B and be like-new, you never know. If you buy in-store you lose the 14-day change-of-mind return window, but of course you see what you're buying (except how long battery lasts)
Potentially knackered battery and scratches on the screen/camera. I would be cautious. Unless you can take from the shop and return it there
Definitely not on par. This has a distinctly mid range processor (Kirin 710) which is about on par with a Snapdragon 660 which in turn is similar to the generation or so previous to the S9 CPU. Think S7 or possibly S8, actually maybe even S6. The camera won't be on par either. EDIT: As you can see here, in fact it is a noticeable step below the SD835 in fact... (S9 has SD845 or Exynos equivalent depending on region). This is benchmarking closer to the SD821 I think, which was on the LG G6 and criticised generally at that time for being behind the curve compared to other flagships. https://www.notebookcheck.net/835-vs-Kirin-710-vs-Kirin-970_8554_10172_9236.247596.0.html
I would have to see it in order to believe it (regular Mate 20 is another proposition, definitely Pro) Would take an S7 over this, or a gorgeous P30 Lite ...but don't get me wrong, this is great value at this low price - were it grade A it'd be perfect, I'm still adding heat
Aha. Yes on par with S9 but not S9 plus. Side by side test don't think the S9 blows it out the water, IMHO.