1999 Volkswagen Golf rear screen washer

Ask him to show you it ? If it has and it's in boot , may be it's leaking or hose off causing water in boot ?If not that , you will have to find where water getting in .

I would ask myself why the salesman has not dried out the boot if it was merely a screenwasher problem. If he knows the cause of the problem, then can he show you the problem in action?
It is possible that the car may have been imperfectly repaired after receiving rear end damage. Look beneath the boot carpet, with a very patient and beady eye, for signs of dents, welds, respraying, gouges in the paint, panel imperfections, badly fitting trim. Check in the same way, lying underneath the rear, when safe to do so, for missing or broken bumper fittings and plastic vent flaps. If in doubt, walk away.