>>1869540You're looking at level and age of components.
>>1869544pic related for example has a mid-tier version of the old style of good STI shifters. So it's moderately desirable.
Beyond that, fit, and you probably want something that has rack mounts in the rear and fits 28mm tires if you want to commute on it. Which is many road bikes.
Condition is probably the most important thing. Drivetrain condition is major. You can visually see when chainrings are quite bad, meaning the entire drivetrain needs replacement soon/now. They wear differently so you can compare to one another. And look out for shark toothing. You might also get a chain wear tool (cheap) and if it's worn to 1.0, that means the cassette is shot too.
Wheel condition is the other major issue. On rim brake bikes you're looking out for rims that are concave or significantly visibly worn down. Also loose spokes and badly out of true is a red flag. Bearing condition also, which is quite hard to gauge just by spinning a wheel. Really you want to remove one (the front is easy) and turn the axle in your hand.
Tires. Tread and look for small cracks on the sidewalls (it's dead). It's ok to buy a bike with bad tires or bad bartape as nice fresh ones will be a major improvement for you, but just don't pay for 'good condition' if it doesn't have good condition tires.
As for derailer and brake caliper condition, try wiggle them. If they have much slop in them they're worn out. Shifters rarely wear out, and can usually be resucitated with some spraying with piss, and poor shifting/braking is usually a re-cable issue (again, it's good to have fresh anyway, so not a red flag). On shifters brake hood condition is probably the big thing. If they're gross to touch it usually means the bike has done high hard miles and well, they're gross to touch. You can replace them though.
The piece of spec to look for on modern-ish road bikes is a compact crank (50/34 chainrings), or semi-compact (52/36)
