Ultrastars are Enterprise class drives and I have never seen an example of a commercial external hard drive product using Enterprise drives. The class, use cases, interface limitations, and price points make putting a designated Enterprise Class Drive inside extremely unprofitable. So it's unlikely they were shucked drives.
That stated, more than likely these are retired Storage Arrays. End of Life NetApp, EMC, Hitachi, etc Arrays that have reached end of support (or reached the 5 year point where continuing support contracts cost as much or more as a new Storage Array) are often taken out by compucyclers, parted out, and their components sold. As already noted, the Smart Data can definitely be overwritten. A couple places to look are in the SMART Logs, as a lot of outfits will fail to erase this data when resetting everything else. If you run smartctl -a to pull all attributes and under SMART Self-Test Log you see big numbers, those are the hours that the drive was powered on for when the test ran. I've seen drives with "0 hours" show over 10,000 hours under the Self-Test Log. This isn't as good as it used to be because of several of the big outfits like goharddrive have gotten *much* more thorough to ensure they're wiping *all* of the SMART data, but there is still use marks and the below that make it very clear the drive is not new, but tampered with.
A really simple evaluation is to look if someone's went and erased the Manufacture Date at the top right of the drive label. If this has been erased, scratched off, whatever, the drive is tampered with, 100% of the time. They do that because many customers won't even look for it, but when they see the drive was manufactured in 2010 or something they'll wonder if the drive is truly new.
More than likely, like almost all these used Enterprise Drives, they're simply drives with 1,000s of hours from a retired Storage Array.
You will have better luck in my opinion buying an openly used drive that may have only been gently used (perhaps a cold spare, or used shortly before an array's retirement), vs. buying "New". If it's "New" and "Cheap" just use your head and remember nothing is free. If it sounds like it's too good to be true, it usually is.