The question is:
How important is your data vs. How much $$$ are you saving?
Many of these drives are likely to be returned for previous problems which may (or may not) have been adequately fixed.
They've also been through at least one more shipping cycle. If you've ever been on a UPS loading dock, you know that can't be a good thing for a complex electro-mechanical device.
There is a reason they only carry a 30 to 90 day warranty vs. 3 years for a new one. If they could make money offering a longer warranty, they probably would.
I wouldn't do it unless it was a hell of a deal, and I wasn't to concerned about what I put on it. At least until it had been in operation for a few months.
In theory, there is no difference between theory and practice.
In practice, there is.