Property with over 50% vacancy

[Osama Mayassi]

Hello,

A property I am looking it is a 12 plex and currently 7 units are vacant. How do I find out why these units are vacant? 

Is it better to stay away from such a deal or purchase a property that is already fully rented?

What is the best strategy to minimize losses from purchasing a property that has substantial vacancy? 

The 5 units that are being rented out currently are bringing in around 500-700 a month. However, the sellers are estimating the remaining units will bring in around 1000-1200. The sellers are old and I am assuming they have had their current tenants for some time.

Any help/advice is greatly appreciated!