Roofing questions?

Is there some sort of treatment you can apply to an older roof to prevent a moss buildup? I've made an offer on a house and the home inspection raised flags about the roof. The roof is about 13 years old with an expectancy of 15-18 years (WA state). We have requested the owner do repairs to the beat up shingles, but I'm curious if there is something that can be applied to combat the moss.