Not being able to rent out units usually just means the price is too high for the market. 

Ive invested in the Boston & DC markets for 22 years now.

The non-conforming duplex was on the market for just $430k.

Go off the % allocated via the county assessor's determination of market value?

What markets are you investing in?  

Alternatively, put it on the market and give the tenants notice keeping in mind that showing times will have to be done with 24 hours' notice to the tenants.

After sending out thousands and not getting responses, or at least not good ones, I stopped trying this marketing method.

There are a lot of affordable properties in Knoxville, but the growth isn't terribly vibrant and the deman isn't what you'd expect for a city of that size and that close to the Smokies - you're mostly looking at college rentals for the meat of the move season, and the SF market, while it exists, doesn't really compare to Chattanooga. 

@Martin Manning To add onto what others have shared, in the most simplistic terms, apartment complex values are based on Net Operating Income, or NOI, and the cap rate.Price = NOI / Cap RateSo, if an apartment complex has an NOI of $100,000 and a market cap rate of 10%, the value is $1,000,000.Listing agents for complexes love to price based on a pro-forma, which is utter nonsense (unless you're in Austin, in which case apartment complexes apparently have floors made of gold).

Study the market.