No warranty covers the product with ITs serial number most of the time.
Not sure what you mean.
WD RMA seems to require proof of purchase and serial number.
Perhaps going outside of these "normal" channels for RMA might get you around these requirements, but it seems unlikely they'd accept RMA for any drive without proof of purchase. Maybe in some cases, but in suspect those would be the exception, not the rule.
That said, who is to say how long a drive sat on a stock shelf before initial sale? An unregistered drive could be secondhand, or just wasn't sold until recently.
I simply meant that I wouldn't assume a used drive includes a manufacturer warranty. I'd work with the reseller to replace the drive, not the manufacturer.