Drop my 5c
1. Geometry. You want spine parallel to edge, otherwise it's an eternal pain to keep straight
2. Smile/frown. Slight smile is fine, any frown is not. Try to find image of new condition and see if it had any smile initially. If it didn't (and it's not a chinese razor), might hint to a problems with pt 1.
3. Cracks in the edge. Definitely a no-go, it might be way deeper than it looks.
4. Chips in the edge. Generally workable if not too deep, but easier to avoid
5. Pitting corrosion. Normally don't affect function unless close to edge (or absolutely massive), but you won't be able to get rid of it realistically.
5.5 Surface rust is normally not very hard to get rid of, but be aware if its too much of it.
6. Edge - generally not a problem if it is parallel to spine, but might take more sharpening than you'd want. Be wary of non-uniform wear, normally sharpening strip on the spine should be more or less same width along the length
7. Wear on spine. Obviously, shouldn't be boxcutter-thin.
