Toilets aren't worth it, because they are like $100 new and the effort to remove the toilet from the wax ring without breaking it and then cleaning off the wax ring to get a good seal when reinstalled just isn't worth the effort. 

Have you ever had them screw it into the window and crack the window or seal in the window?

We did end up having to do some painting and sealing on the new tub and shower, but those items the seller had to do saved us thousands.

Yes, it will be enough to seal the deal, but what about deferred maintenance, renovation, and other projects?

It will seal the smell in and leave the place smelling fresh.8.

I have read that I would need to carpet, paint/seal the walls, and clean the ducts.

Everything got tuched new paint inside and out, referbished kitchen, new bath, buffed and sealed floors, new lights, everything was touched.

They also mentioned they would seal the replacement pipe with some kind of "sealant" so that tree roots couldn't get in their again.My friend was also planning to build an accessory dwelling unit (ADU) in their backyard.A few questions:1) Should my friend have the sewer pipe replacement done with the ADU construction (thinking it might result in a lower overall total cost for all the work done) or should they hire out the sewer replacement job separately?

Is this a seal deal?

Look into sealing your rim joists.