So I called (for science!) and it's good news/bad news I guess? Good news is they stated that as long as the product was purchased new the warranty is for 5 years from the date of purchase--the serial number is irrelevant to the warranty period. Ebay sales are also purportedly ok, again, so long as the product was new and not used. The less-good news is that when you make a warranty claim you need to provide your proof of purchase and who knows how they're going to react to an ebay sale from "horrorgrl666". I'm going to keep my original packaging just in case I need to make a claim though. Based on my reading of
the warranty I
should be fine--I am the "original end user customer", and the drives were not "sold as second-hand" or "resold contrary to U.S.A. and other applicable export regulations."
But as with most things, there is a YMMV here, so potentially be wary if the availability of warranty replacement is of paramount importance to you. For me, the capacity and performance for the money is worth the fairly small risk that one of them dies and I can't get warranty support.