Test Drove and put down a deposit, sale contingent if they can fix the rough idle/hesitation/skip causing what feels like car running on 7 cylinders?

No CEL light.

Any ideas? Car was washed last night....not the engine compartment. Not sure what kind of gas they put in the car either as it was on Empty for quite some time before they prepped to sell.

B6 S4 Cab - 66,000 miles - well maintained - new clutch/tranny/radiator around 25,000 and 40,000 miles....

6 speed MT