Tell tenant frankly that pet smells and you are concerned about getting complaints from other tenants (as if you haven't yet) and clean up costs for pet odor permeating walls/surfaces/floors etc. -- which you want to avoid to avoid having to deduct security deposit charges (or increase security deposits next time around in order to pay for pet cleaning/deodorizing).

Mix TSP and water to scrub the walls, it'll pull the nicotine out. 

The smoke damage is quite substantial with nicotine staining on the ceilings, walls, and AC vents.

Nobody will know why they led you to believe they would sell outright, but sometimes our why we landlord permeates through to why we sell.If it were me, I might find the type of buyer they are looking for (Like in my own portfolio of tenants) and try to make it a win-win.  

Any other ideas to help prevent the smell from permeating our unit? 

It gave me a headache but rental income is a great motivator so I sucked it up and marched ahead.As you can see in the pics the walls were covered in nicotine.

The hallway between the kitchen and the garage didn't have a heat vent so in the coldest part of the winter (this was in Ohio) the warm air would condense at the ceiling and deep orange tar and nicotine would drip down 🤮

Cleaning the walls, the nicotine looked like ice tea when it hit the floor.

There is no damage, it's water vapor not burned nicotine from smoke that stains and odor lingers.

Smoke permeates EVERYTHING.