Before selling us your DSLR or mirrorless camera, check the shutter count.
Depending on the brand, you’ll find the correct method to do it.
For Canon, Nikon, Sony, Pentax: https://www.camerashuttercount.com/
For Fuji: https://www.apotelyt.com/camera-exif/shutter-count-finder
For Canon (Mac users only): https://apps.apple.com/it/app/shuttercount/id720123827?mt=12
Button procedure directly on the camera body
Panasonic GH5
Panasonic GH3 / GH4
Panasonic GX-80 / GX-85
Panasonic G9