The RS1200 was one of the earliest Buell motorcycles offered, built long before Harley Davidson had any interest in the fledgling company. Based around the 1200 Sportster engine, the RS1200 was a far better handling bike than any Harley, but it really needed more power. The owner of this Buell took care of that.
The seller says:
This engine alone has had over $10K in upgrades! Sputh Crank, Aluminum Jugs, Coated Pistons, Andrews V8 Cams, STD Race Heads, Roller Rockers, Shorty E Carb, MSD Ignition, ...
The bike's almost worth buying for the engine work alone.
I personally owned one of these RS1200s and this one already has the later Corbin seat, a welcome improvement on the original, plus, as noted above, the engine work has been done. I would guess, during that process, the front engine mounting bolts have been replaced to remedy weak studs used in the early production run.
If you want a very clean example of a pretty rare early Buell, this is worth a serious look. It's for sale on eBay.