It is probably not "what info you need to get from the seller", but rather "what do you need to check for yourself".
For used lenses, here are some areas that I'd normally check:
- Look though the lens. Any fungus or defects on the glasses? How bad is the dust?
- Can it mount & unmount to/from the camera smoothly?
- Is the zooming smooth (if it is a zoom lens)?
- Is the manual focusing smooth? Can it reach it's full range?
- Is the aperture working properly? (you can use the DOF preview with various aperture setting to check)
- Can the lens communicate properly with the body? Any contact problem?
- Any problem with the filter mounting thread?
- Take some sample shots at different focal length. Is the image sharpness satisfactory?
- If it has built in motor/image stabilizer, are these working?
- Cosmetic: any dents?