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 & 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).
It's not so much a problem with an App that can get this information (SMART info), it is a problem with the USB to SATA controller itself providing the necessary interface to actually get the info. See this relevant answer for reference.
Your best bet is to get a Thunderbolt to eSATA adapter and just use the built in commands on your Mac:
diskutil info diskX | grep -i smart
What I do is I have an old Dell PC for testing out different components. I have FreeBSD installed on a small drive and use the second/third SATA ports for testing/cloning drives.