So you really have to do nothing since you are an individual and not a car dealer but if it was me and to be really nice I would offer to fix the window leak since it may not have been rainy the day he test drove it. Since he test drove it he should have noticed any brake problem so I would not take any liability for that or anything he should have noticed while test driving. Maybe offering the help on the leak will be enough to get him settled and then he may be happy enough.