Judge size together with resolution
A bigger screen doesn't automatically mean a sharper picture. Compare the size with the resolution and how far you sit from the screen. For office work, readable text and ergonomics matter most; for gaming, also the refresh rate and what your computer can drive.
Find the full model code, as similar names can hide different panels and connections. For colour work, a marketing phrase like «vivid colours» is no substitute for real specifications or measurements. A review of a new monitor proves nothing about the condition of a used one.
Check the screen and what's included
Look for information on dead pixels, blotches, scratches and uneven backlighting. A photo of the desktop may not show every defect, so plain single-colour test images help. With OLED screens, also check for burn-in.
Make sure the stand and the right power supply are included. If you'll use a monitor arm, check the mount is compatible. The HDMI, DisplayPort or USB-C cable you need may not be in the box, and the supported resolutions depend on the connection you use.
Compare the whole offer
Weigh the monitor's specifications together with its condition and transport. Delivery needs proper packaging, so check on the lot page how you can receive it. A cheap monitor without a stand or power supply can cost more in extras than it first seems.