macOS app for showing external HDD usage statistics? I'm planning on selling a used external HDD on eBay.  Are there any applications for Mac out there (preferably free) that can show me statistics about the usage history for the drive so that I can let potential buyers know the condition more precisely?
By usage stats I'm assuming you mean SMART stats...

DriveDx has a trial version that will show SMART stats. But you have to load a kext to see USB attached disks. That's a common issue with all the 3rd party programs. 

It works well, and will allow you to do short &amp; extended self-tests even in the trial version. It will save a report you could email to your potential buyer (Actions | Save Drive Health Report).