Also will likely have to rip out all the sheetrock in the affected areas and treat all the framing to kill the smoke smell.

I wouldn't consider that a smoking hot deal, but maybe doable?

There have been cases shown and decided that a "chain of events" that led up to the closing and sale had the buyers broker / agent earn a commission.So a lot of it is subjective as to what happened, how it happened, and at what time.If this is a smoking hot deal and they are going with your offer and it's a low sale price then another option is to let them double end it and get a real estate attorney for 200 to 300 to negotiate the purchase and sale for you.

However, there has been a fire previously and there is smoke damage.

The chiminey itself still functions great from the aspect of building a fire and getting smoke out.

The only secret is in the minds of lenders in sizing up a borrower as to character, ethical aspects, intent, and if someone is blowing smoke at them.

High sheens show wall imperfections more but allow you to better wash off the grime, smoke smell, pet stuff, etc that may come!  

If the plumber they used went in there and just snaked it, that would have been all they had known.Unless you can find some sort of smoking gun proof that the former owners were told by a plumber that the drain pipe was put in wrong and needed to be fixed, I don't see any way they could be held liable.Unfortunately, its just called a piece of bad luck.

@Tory Ellis ...welcome aboard and good luck on the first deal you got going.There is a reason they call them "GURU"....all they do is blow a whole lot of smoke..See you around the site.

That stuff i would need to install, probbaly a coupole more smoke alarms also, but not worried about that.