What is your lease situation? The lease transfers. Your LL cannot evict you for reasons of selling the house.

If you are month to month you still get a 30 day notice which technically is 30+ remainder of month (so if he let you know today you would have to be out 1/1 not 12/21). Did he give you notice that he needs you out?

If he wants to buy you out of the lease, then make him pay. I don't know what your rent situation is, but even after rate increases he's probably in for making a bit of cash here and has incentive to sell sooner rather than later. So you could ask for a decent buyout here - 6 months rent or something plus moving expense. Something like that.

Or, he could sell with you in it and then you have to negotiate with new owner. Do you know what this will look like already?

But the biggest question is your lease and what its term is.

I wouldn't recommend outright blackmail, it will get you in more trouble than you think. But there's nothing stopping you from telling any people that look at the house about its problems, especially if they are doing a walkthrough with an inspector "hey, don't miss this!" whatever. I'm not sure what would really come of this though.