If you have concerns about the tenants that are there, you can ask the owner to have them vacate as a condition of sale, but at the same time, he could tell you to go pound sand and back out of the deal.

(guess) of their listings I think everyone should call their bluff and tell them to pound sand. 

This takes a LOT of my time.If I went to get a judgement against this tenant for the balance of the lease (if I could even find them to serve court papers), the judge would tell me to go pound sand, absorb it as a business expense, go re-rent the apartment, move on with my life and quit bothering him with this "nonsense."