I've bought a few, and haven't had a bad one yet. I had a 3tb drive crash, but I'm convinced it was the dock I plugged it into and not necessarily the fault of the drive (which tested 100% healthy before and after formatting prior to plotting, and then crashed during plotting).
One fool put a 2.5 (not for plotting) in a padded envelope and then didn't mark fragile on the outside... It dropped a good three feet through the letterbox onto hardwood floor.
Just make sure you ask your drives be marked fragile. Seriously... that was a one off and more the fault of the parcel deliverer.
Previous feedback is a pretty good metric of how well your item is going to be packaged and in what condition it's in... it's all there on each sellers profile.
Happy hunting :)