If you ever bought a car that was owned by a smoker, you probably know how difficult tobacco odor can be to remove. Smoke particles are very fine and can make it through a lot of different filters, and there will also be a film left behind from the tobacco that is very hard to clean up.
While this is a pain when it comes to vehicles, it is even worse in a home, so before buying a home of someone that smoked indoors, you should consider if you are ready to do the work necessary for cleaning it up.
While some claim to have had success using ozone to clean up tobacco odor, this is a dangerous gas to be dealing with, and it could be lethal if you are exposed, so it should be used with care.
The only proven and true method is to clean the house thoroughly. There is not going to be a single part of your home that can be overlooked. Even light fixtures and lamps need to be cleaned off, since the heat from the bulb will make the odor airborne again.
Any filters that you have in your home need to be replaced, such as, in HVAC units and kitchen fans, among others.