.: I just bought a 2016 house that has a few missing shingles. 

When repairing an existing home, there always seems to be some sort of a surprise... termites, underground plumbing, lots of wood-rot under the shingles, you name it... 

Shingles from Lowes was about $1200.

We get to the job and the shingles are on a pallet in front of the garage .

In the old days, grandpa would buy the shingles and spend his week of vacation from the factory replacing the roof when it was needed.

They don't assign much, if any, difference in value for a 10 year old shingled roof versus a 20 year old one. 

You go metal right over the shingles.  

Oftentimes, mobile homeowners will roof their home with a shingle or steel roof for added visual appeal and water protection.

As a GC I can advise try to retain control of your finishing material orders (anything where you need to make a choice ((tile, flooring, lites, faucets, tubs)) vs. construction materials (2x4s, mortar, shingles)) because we get those discounts on the finishing items and we keep them!

If I was your buyer, I would have liked to see an accent color on the shingled areas under your roof gables, but again a lot of this is personal.