I once paid good money for a good inspector (very highly recommended and reviewed in the area - used by lots of realtors, etc.) who completely missed three pretty significant items - one of which was that the roof had FIVE LAYERS of shingles.

I knew this was going to happen from all of the content I had consumed, but I had no idea how many there were going to be, or how important they would be to fix immediately.For a little more context, the house I bought was built in 1910, and within these first six months we have had flooring issues (the kitchen floor almost became the basement floor), pest issues, furnace problems, electrical issues, contractor problems, shingles blown off the roof, and more.

See if needs to be upgraded.6TH Check all windows.7th Roof look for lifting or sagging shingles ALL the best, 

@Chris Lo In New Hampshire that’s what Cedar shingle houses look like as they age.

It is a computerized selection when that happens.  

"shingles" will be 15-22yrs. 

Shingles sometimes need to be replaced, but an entire slate roof failing?

GFCI are safety/liability issues specially for rentals.About the roof sagging, it happens, I will be more concern about multiple shingles layers, you will need a new roof covering, get a roofing company, sometimes they wont even charge for the inspection as long as you hire them. 

As far as future upgrades, likely re-shingle the roof, install better soffit vents (can be done from outside) and might consider replacing all of the windows, crack sealing and weather stripping.

If that's the case then why even have a real estate industry.. why not just be like China were no license or industry exists anyone can hang out their shingle.