Many disk failures are attributed to the disk firmware predictive error checking flagging the disk as bad. Unless there is a head crash or physical problem the data is still accessible.
You are correct to make the assumption that the disk might months later show up on ebay.
If this is a SATA disk, consider using the disk's firmware level secure wipe function. It's been included on all SATA disk firmware since 2001.
UCSD’s CMRR to the rescue
The University of California at San Diego hosts the Center for Magnetic Recording Research. Dr. Gordon Hughes of CMRR helped develop the Secure Erase standard.
Download his Freeware Secure Erase Utility, read the ReadMe file and you’re good to go.
To use it you’ll need to know how to create a DOS boot disk - in XP you can do it with the “Format” option after you right-click the floppy icon in My Computer.
You may have to dink with the bios on a desktop PC to get it to work properly. Most motherboard manufacturers have blocked the function because of course it is dangerous.
They typically do this by locking the drive access during post, you can get around this with the right bios settings and waiting to connect the SATA data cable until after post is completed.