Do they have a pet, or use candles and/or air fresheners?

I did this once (in writing saying as long as there are no damages that would be okay.......) and I’ll never ever do it again.... they left the entire place full of belongings, trash, dog poop, broken glass, cupboard knobs in garbage disposal, wax warmer wax down the sinks, and much much more......

Ultimately I decided the best option for me was simply a simple reed oil diffuser that's perpetually on sale at Walmart for $5 bottle, lasts 30+ days, and does not look like a those cheap dollar store twist-up air fresheners that looks like a cross between a toilet wax ring, a fly trap, and a skyscraper Smurf house.  

Only one out of 4 fix and flips had a granite counter and the other 2 had the plastic like counter that I think you seal with wax.

I wish you luck, I'll cross my fingers, hell I'll even light a candle for you this Sunday.

.”― Carl Sagan, The Demon-Haunted World: Science as a Candle in the Dark

Now you can get into retail centers and that is a whole other ball of wax to deal with. 

I tried flipping houses about 10+ years, different ball of wax I know, but I didn’t incorporate and it ended very poorly, so I’m trying to protect against things of that nature.

Many things can leave smell or residue (cigarettes, marijuana, vaping, candles, greasy food fry).

Whether it is pot, cigarettes, vaping or even candles; there is damage and odor that can occur.