Some obvious things, some less obvious;

1) Visible rust.

2) Hidden rust.

3) Visible rust that is meant to draw your attention away from hidden rust.

4) Mis-matched tires (maybe a little pedantic but if I see 2 goodyears, a michelin, and a bf goodrich on the same car I’m going to have some questions. . .)

5) Paint on the door hinge bolts, if they’re painted then the car has been resprayed at some point.

6) Missing/out of place accessory bolts under the hood, etc. with no explanation (clearly someone took it off for some reason and I’d like to know why).

7) A brand new part that was clearly installed recently but the supposed single owner has no clue how it got there.

8) Any problem that was not included in the original description/ad but is present upon inspection.

9) Pictures taken in the winter but the ad was put up in the summer. . .(or vise versa).

10) numbering mistakes like I made in the previously unedited post.