@Chris Donathan, Sorry, I thought I replied to this the other day. BP saved my comment, I just apparently never hit submit.
It's my understanding that if the house is zoned residential, it must be owner occupied to do short term rentals and you have to obtain a permit. If it is non-residential, you can do non-owner occupied, but you still have to obtain a permit.
I am aware of individuals who own non-owner occupied short term rentals within the city limits, but I’m sure the city is aware of them too, after all, you have to advertise your home online to have any guests. I’m not sure about how the city is going about enforcing the ordinances other than sending out letters and fines.
The city codes will have some changes soon concerning short term rentals, but it’s mainly about accessory dwellings and the property would still need to be owner occupied. You can watch for updates on this at https://recodeknoxville.com/
If I were looking to buy a short term rental in the Knoxville area, I’d look just outside the city limits in Knox County where it's still the wild West or possibly in the Sevierville/Pigeon Forge/Gatlinburg area.