I have exactly the same problem , the ones on Ebay are not only aftermarket, but also a different part, that being VAE06 opposed to VAE01 of the standard ISCV.

The way to tell is the genuine one has this written on it, and will be expensive!

By not working do you mean the engine just runs too fast, as this is what happened to me?

James (MSD) gave the game away, it's to do with the spring ratings.

I dismantled both of mine and noticed the genuine one has a stonger bottom spring than the aftermarket one.

Thus the valve opens more=high idle, I tried swapping the springs etc but it was never right, I still have it and may have another look at it at some point.

Martin