I would make certain I followed proper security deposit procedures, documenting everything showing you used the whole security deposit in back rent and damages, then send it certified mail in a timely manner to his last known address, even if it is that unit.  

The best open rate is going to be on hand-addressed, first-class-stamped pieces.

@Josiah MosleyAs @Rhett Tullis mentioned, this appears to be only half that duplex.  7709 NW 113th Pl, Oklahoma City, OK 73162 is the address of the right side of the house according to Google.The 1 car garage was my first hint.Your thinking is absolutely correct.  

I would send a real, handwritten letter and mail it to her if you have the address.

I like Roy's disclosure of each pointing out the benefits and down side, be careful with the downside and not really addressing that side fully.

As an example: John Doe, realtor, home address, age 46, date of birth, married, likes watching football on tv, golf enthusiast, 3 investment properties, do it yourselfer, 60% loan to value, $650K home value, length of residence 21 years,  NOW...if you are selling a do it yourself putting green....he might be ideal lead.There are many list providers out there for realtor email lists...but if you know what makes someone tick...that is where real value is.

I posed this question on a recent forum post and got this response:To address your question we have to pause on the idea that you implied of "perfectly good".

Also try to plug the address into Google sometimes it will pull up who did live there.Good luck

hi rafeal. i have a number of different ways to find the owners, usually they are free  also. first, find the house, then check with the local tax assessors office. they may have a new address for the owner. second, county websites often list the owner. most often, it will list the owner as living in the house, which we know is not the case. then, if you get the owners name, check the website for any other houses he or she may own in the area. chances are they live in one of the other houses. if they do not own any other houses, run their names on whitepages.com. you might be surprised with what you come up with. as a last resort, i knock  on neighbors doors around the house. its amazing what neighbors know. 

Also, be sure to check out the Podcasts and Blogs for posts that address the subject.