If the seller says 'no', then it's up to the buyer to walk away or take care of it themselves.

6 is not that high though. Sure, it's above the 4 mark which is viewed as the level when something ought to be done, but unless you spend the next 10 years in your basement, then there is probably a minimal risk that you'll be affected.

That said, I'd still take care of it, it's just not an emergency that needs to be taken care of today.