I'm not so sure a state qualification would be your biggest concer. To provide a roof "certification" you are putting your reputation and business on the line because you will be stating in writing that the roof is sound, free of leaks. Some buyers may want a guarantee/warranty that states the roof will last at least another year or more. So I would check with your insurance carrier!
Thanks for your question.
Check this directly with the town officials of the areas within you are interested in working. You have to be licensed and insured. If I needed a roof cert, I will ask for that. Then network with realtors that are in those areas so when they have home selling they can reach out to you.