I did buy it from the same seller, yes. I had no choice, as they were the only one (and I assumed that I had ruined the first one, rather than it being dud all along. I never did try it without sanding, though, so I don't know for certain). I'm certainly not buying that one from America; international shipping is $29. That's utterly ridiculous.
The thing is, I don't want to assume that the ZIF is dodgy and go and order another one unless I'm absolutely certain that it is the ZIF that is the problem. I might order it and just end up with the exact same problem as before. That's why I'm so stuck about what I should do - I have no way to be absolutely sure which component the problem lies with.
I thought the ZIF cables were pretty simple, anyway? Just a couple dozen flat wires all running parallel. Surely it should either work properly or not work at all, rather than sort-of working as it is with me (as there very definitely is power getting to the hard drive, but it's just not spinning up properly or being recognized by the player system).