Doing a little research, in case it might save you some pain:
Searching ETKA on that part number, it says suffix "A" is superseded by suffix "B", which in turn is superseded by suffix "D", but that all are interchangeable (reverse supersession).
ETKA says the 420 615 301 D is the correct part number for 2008-2012 R8 front brake disc. That part of the ad is correct.
The p/n for a 2006-208 RS4 front brake disc is 8E0 615 301 AB. The first three digits usually signify the model, so 420 (R8) vs 8E0 (B7) can be explained that way. The "D" (R8) vs "AB" (B7) suffix, not so much.
ETKA PAL search capability says the 420 615 301 A part number applies to 2007-2012 R8. Substitute the "D" suffix, and it adds the 2010-2016 RS5, as well as the R8. It doesn't mention our RS4 at all.
The picture in the ad is for a rear brake disc.
Our rear brake rotors are attached to their hats using pins, as illustrated in this image, like the picture in the ad:
http://www.stokeaudi-parts.co.uk/product_images/w/068/Picture_007__63535_zoom.jpg
The front brake rotors on our RS4s are bolted to the hats, as illustrated in this image:
http://www.stokeaudi-parts.co.uk/product_images/n/309/Picture_006__62183_zoom.jpg
If you can confirm with the seller that the rotor is bolted to the hat, then at least you have a fighting change.
Hope this helps. I can keep poking in ETKA if you have more questions.