That's definitely nicotine stains. Any solvent worth it's weight to try to remove it would not only potentially destroy the integrity of the knob's compound, but would probably be futile anyway because nicotine is of course an oil and by looks of it, has permeated the knob's surface more than surface deep, along with tar. Replace the knobs, but you can count on the rest of the A4's surface outside and inside to be as likely polluted.
Also be wary of purchasing equipment from people that vape. The nicotine is still very much present with the added displeasure of the glycerin slime from vape juice. Nasty habit.