Not in the market for your camera, but if you're looking for a free piece of software for Mac to check shutter counts, google "EXIF Viewer"
Follow the install directions, and once it's open, you can take any straight-out-of-camera jpg (but not raw) and get complete EXIF, including shutter count.
Keep in mind that only 1-series cameras record shutter counts in their EXIF, the 5D and everything below it do not record that data.
Good luck with the sale!