macOS app for showing external HDD usage statistics?
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).