In the case of things that wear out / have a finite life, I don't think I'd buy used, at least not without having a jolly good look at it. It's fine for things like trim or brackets or whatever that last indefinitely but as you say, you know nothing about the condition or how much life it's got left in it.
I think I'd probably go for the eBay one, provided you're satisfied it's the right bit. The seller has 100% positive feedback and the price is excellent. For all practical purposes I reckon a third-party wishbone is likely to be just as good as an OEM one.
Just one thing - you said you need the NSF wishbone, but the one on eBay is described as RHS. I think you still drive on the left in the RoI, don't you?