[Walt Fricke]

If a torsion bar broke, you should be able (or the current owner should be able) to get a used one dirt cheap. Most owners of 911 who get bitten by the track driving or autocross bug end up installing stiffer torsion bars. The old ones sit in garages and basements, as there isn't really a market for them.

Replacing a rear torsion bar requires removing the rubber strip along the rocker panel, and then removing the separate rocker panel (horizontal strip under the door)), and then the oval plug back in front of the rear fender. Then, after disassembling some of the rear suspension, the old bar can be removed, a new one installed, things put back together, and the car aligned. This is not a trifling expense, even assuming there are no problems getting the inner broken piece out. Which underlines the virtue of the comments which suggest you get the current owner to pay for the repairs, or else discount the buying price by the cost estimate you have gotten from a local Porsche dealer or independent shop for the fix.