So I bought another digitizer from the same seller. Same as-new condition, except there was a minor flaw. If you look at the pictures below, you'll see that the speaker design is different in the two screens. One has two tabs and the other has two springs. Now the digitizers in the picture are the OEM ones, and the N900 frame actually has 4 sets of pads to accommodate either the spring or tab design. Sorry I did not take pictures of the Ebay digitizers, but they look identical to the OEMs, down to the printing, sponge gasket, colour and everything else.

Ironically I received two different digitizers from the Ebay seller as well, the first one with tabs and the second one with springs. On the spring Ebay digitizer of the springs was completely bent, however given the tight space between the digitizer and frame it still makes perfect contact. And it is far more sensitive and accurate than the OEM.

I've tried the test you suggested, and nothing much happened with four different digitizers (2 that came off the phones and 2 Ebay replacements) EXCEPT that when rotating the colour would shift from green to purple at certain angles. So that might mean the Ebay replacements are polarized as well.