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

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.

@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).

Diversifying your portfolio can also be beneficial, as it helps mitigate risks associated with market fluctuations.

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. 

Study the market.

The secret to dominating the world of off-market acquisitions isn’t about delivering a slick pitch or being pushy—it’s about becoming the trusted, go-to problem solver that sellers want to work with.

In our market (New Orleans) cemetery tours are very popular. 

Crexi, Loopnet are probably the two biggest but why not work with an agent that can present you with multiple off market and listed deals?