Believe me, it is a nightmare.

Difficult situation for sure.

This is not so uncommon!

Raoul, 

I agree with Scott and Annette. Helping them find a new place might be the sellers best answer.

Carrots are going to be your best solution. Possibly offering a discount on rent to assist in the upkeep of the property has helped many of my sellers in the past. 

However, if the buyer is wanting a rental, having tenants is a good thing. So, bottom line who is your buyer? 

Repeat so they remember, then retract the offer you extended that was conditional on their co-operation. Perhaps your carrot did not have sufficient substance.

Advise owner a sale can not occur until tenants are gone. All you need is 30 days to sell, 60 days to close. Unfortuneately, the owner may be facing eviction proceedings. 

The best course is finding a solution via the tenants. Find them a place to move. Increase the benefit of cooperation. Find the reward that motivates them.

Let the seller know.

I bribe my tenants with a 500 bonus if I sell and an additional bonus if the buyers want them to move.

If the tenants don't cooperate...tell the owner. Then hopefully the owner will get rid of them. 

I find if the tenants will have top move, they never help. If they have a chance to stay, then most will go out of their way to let us in, be clean and polite.