Do you mean just the diffuser itself or with the bottom part of the bumper?
I recently bought a S3 diffuser on eBay which was totally rubbish and loads of scratches and stone chip... so I just returned it to the ****** seller whom described it "As New" was more like coming from war or something...
So my whole point to you is to buy a brand new diffuser, but you could get the bumper "USED" as you would re-spray that anyway, but with diffusers you can't...
So don't waste your time looking over eBay, and then ending up returning it to the seller...
If you look at my S3 conversion thread you'll see the parts that I'm after and still couldn't source any used good ones!
The only good part I was able to buy so far was GENUINE S3 Spoiler...
But for other parts I'll defo buy BRAND NEW direct from Audi
Just waiting for some cash as I just bought 18" BBS CH, probably the coming month... for other parts
Hope it helps