Headlight units are prone to leakage, where water gets in and can't get out. The lights continue to work, but the sloshing and fogging bother many owners. Dealer price for new units is $700 per side. Some owners report loosening the sealant with a hairdryer, then re-sealing with silicone and reinstalling the lights.
I'd get permission to wash the prospective purchase and make sure the lights stay dry inside!