The Low-E coating had failed and began to oxidize causing a metallic "cloudy" look which the buyer's asked to be replaced. 

 @Joel Owens,Don't hold back, Don't sugar coat it.

Option 3) Finally, it has crossed our minds that perhaps it would not be a bad idea to do flip for our first deal on a home that needs basic and minor cosmetic updates (new coat of paint, laminate floors to replace carpet, updated appliances and cabinets perhaps).

You may be able to contact animal control and have them pay a visit to the owner if you don't want to confront him/her directly. 

Yet the buying out of state is a whole different animal.

Having your vent this close to the ground is inviting not only kids to put things in there, but animals as well.

You were supposed to have issued him a notice of non-renewal if that was your intent (not to renew).It doesn't matter that he is a vet, or that he has a service animal (both situations can be given notice of non-renewal without consequence if done in accordance with the landlord tenant act, and your local laws).So, check what the rules/laws are for your area, and issue the proper notice of non-renewal.

My 2 key questions in my ad are How many animals?

Usually 1 or 2 coats of kilz or zinsser will usually do it.

There may be an ordinance that stipulates what a dog owner / controller must do when they animal does their business.