Thanks to everyone who have provided precious advices, I am now focusing on getting new tenants rather than dwelling on the past. 

Property management can take up your precious time that you should dedicate to your other business.

My condos have doubled in appraised value, and in addition they now net a combined 41K per year which is, for me, precious supplemental income.

I invested in precious metals and made 8% in 6 1/2 months.

@John Smithe - This is going to come down to the following:1) Your personal investment philosophy: your goals, your interests, your time horizon, your risk tolerance, and your interpretation of world events 2) Your unfair advantage: your skills, your expertise, your network, your resourcesBiggerPockets tends to bring together like minds and similar experience, so this sampling bias might result in concluding that a large multifamily in the Midwest, for example, is the best use of your funds.Ask the broad universe of investors the same question, however, and your answers will be quite different.Real estate might well be your best investment, but so might be starting a business, or picking stocks, or buying precious metals.  

Real estate, precious metals, perhaps even a bit of cryptocurrency if you are feeling froggy.

Note to any magazine article writers that hate landlords: I am advocating following all state and local laws to "protect" any and all precious tenant victims. 

I'd also check the county records (mine are online) to see if he sold it.Remember, your time is precious. 

Wasting a month or more of their precious time while you look for and end buyer could destroy them financially and give you a bad reputation, so be prepared before you go in, knowing that you can ultimately help the seller solve their problem. 

Your time is even more precious than money; money can be replenished but those "six/few hours" are gone forever (bet you could have found something better to do than clean up his trash).