If you're buying a used 360 camera (or even a new one), you need to check the lens carefully. Here's something you absolutely must do to check the lens for scratches.
A scratch usually won't affect the image quality of conventional lenses, but they will have a noticeable impact on 360 camera lenses. This is because with conventional lenses, scratches will usually be too far out of focus to have an effect in the shot (unless there is a large area that is scratched). However, with 360 cameras, the focal length is very short, and the depth of field (the area that appears to be in focus) is very deep. As a result, a scratch in the front element will be much more visible in a 360 camera lens than in a conventional lens.
A recent personal example is when I scuffed my lens on my Theta. The scuff was very light but it dropped the sharpness considerably.
One of the ways to check a lens is to use an LED light, which can reveal scuffs and scratches that you might otherwise not see. You can use an LED flashlight, or even your phone's LED flashlight, at different angles.
You can see the effectiveness of an LED light in these shots. In the photo above, you can see only one scratch. However, with an LED light shining at a different angle, you can see the lens has many other scratches.
|This side of the lens has several scratches.|
|Besides scratches, this side of the camera has spots which are chips in the lens.|