When I buy used I am specifically looking for something I can't get new, so I went out and got the old Martin acoustic and a 60's Fender teles. So for me it's more about "what do I look for when I shop for a vintage, rare, or collectible guitar?"
Of course I don't worry about scratches and dings because it's hard enough just to find one. Used guitars for me are either vintage or nearly vintage. If I am thinking of buying an RG just a few years old, I would rather save up the few extra hundred and get the thing brand new from Rich.
So in very limited context of what I go for used (hard to find stuff that is long discontinued), I only worry about the 1) basic mechanics and 2) electronics.
Is the neck straight and does every note play? Are the frets usable or do I have to refret and would refretting destroy its vintage value?
Do all the positions play and could it work with original wiring and am I willing to replace wiring knowing that on some very special guitars that this can hurt the value? Of course I wouldn't mess with a 50's Gibson in any way, but if one precious PAF was simply not working due to broken solder joint, I would take it to the most expensive, famous luthier and have it fixed. But if said wiring was only scratchy then I would leave it alone. If it's a vintage piece and some frets are less than new but still kind of get the job done, then I leave it alone. Jimmy Page played the hell out of his #1 vintage Les Paul and while realizing that he could make it play much better with a full refret, he only dressed the frets as little as possible and replaced one or two that couldn't be reclaimed. And relating to prize Ibanezes, I came upon a Jem formerly owned by Steve Vai or Iceman used by Paul Stanley, I wouldn't change a thing, maybe even the strings. At a certain point some guitars are a piece of history and shouldn't be changed.
Check this out:
I would want to get proper 3 way switch, period correct jack plate, two more original and matching knobs, and OEM Kluson tuners but those idiosyncracies make this guitar what it is in rock and roll history.
But if I was thinking of buying an inexpensive, fairly replaceable, and otherwise used guitar, then I would add the often overplayed factor of 3) cosmetics.
Does the thing have too many dings? Is the pickguard broken and could I replace it? Is the bridge and parts pitted and could I replace those with original or somewhat similar aftermarket model? Will the existing electronics be easy to mod out and add or subtract stuff?
I assume most buyers of used stuff are in this category of getting something fairly common so then cosmetics is as important as anything else so get the cleanest example you can find after looking at several specimens.