The buyers that are left are serious people who get all of my time and effort.   

Since I was deployed so much we focused our efforts toward building passive income. 

The Landlord is obligated to make a good-faith effort to find a new renter which will effectively terminate the lease and any obligation your niece has.

But even thought it did not work out in that case, that same Realtor brought me a deal shortly thereafter, so it was still a worth-while effort.

Run a rental report that shows the seller / agent that this property won't cash-flow above your offer price (or won't cash-flow at a rate you find worth the risk and effort).If they refuse your offer, then make a counter offer closer to their asking price but with some incentive present for you, like some amount of seller financing that lessens your out of pocket expense, thereby increasing your return even at the higher price.

Build your holdings with tax sales.While for many the allure of real estate investing is passive income, it’s hard to make money without effort.

There are exceptions to this rule, but it takes effort and patience to get something good.

I make an effort to get to know the neighbors around my properties and I make sure they have my number. 

They want out, but if they leave they've wasted years of time and effort because they don't get the rentals.

Your response got me concerned that its a huge gamble and not worth the effort or risk.