Everything that decides what a record is worth is either on the label, etched in the run-out, or visible only when the light runs across the surface at an angle.
The three asks
Tilt the disc into the light. Scuffs, hairlines and a cleaned-but-still-marked surface all appear this way and are invisible straight on.
Show the label, both sides. Pressing, catalogue number, variant.
Show the run-out. The etched area between the last groove and the label carries the matrix markings that identify a pressing.
A seller who does all three without being asked is telling you they have done this before.
Then ask the obvious question
"Have you played it?" If yes, ask what it sounded like. A seller who has played a record start to finish and says so is giving you the one piece of information a camera cannot.
Warp
Ask them to hold the record edge-on and turn it. A warp that makes a record unplayable is rarely visible flat.
Grades, and what they mean here
Grades are written disc/sleeve — "VG+/VG" means the disc is better than the sleeve. If a seller gives one grade, ask which half it refers to. The full ladder.
If it arrives wrong
A disc that skips where the seller said it played, a sleeve that was not described, a reissue sold as an original: all of those are not-as-described, and buyer protection covers it for seven days from delivery.
Questions people ask
Is a reissue worth buying?
Often, and sometimes it sounds better. It is only a problem when it is sold as something it is not.
What is ring wear?
The circular mark a record wears into its own sleeve over years. Cosmetic, sleeve-side, and should be stated.
Should I bid on a record nobody has played?
You can, at the right price. Treat "unplayed, ungraded" as a lower grade than the seller is hoping for.
More: how bidding works, packing records.