not quite sure i would say that driveshaft splines would be classed as a wear and tear item!!!battery,tyres,clutch,drive belts,brake pads/shoes,cv joint yes,but not a driveshaft spline

i would imagine where i work we would be offering to put the car right at our cost,regardless of what it cost us unless it was going to be very expensive and if this was the case we would offer a full refund.
we have had a few issues where we have sold some of our p/x's and they have come back needing major items and we have either sorted it and lost money or given a refund-
its a percentages game as 90+% of the cars we sell they make good money on-cars are cars and some will always be problematic!