Obviously it's not infallible. But even if the owner only cleaned their stuff now that they are selling it that still tells you they are the type of person to take action where it's beneficial
Gross house means almost certainly not maintained.
I don't think so, esp at selling time, I definitely expect that the owner has just made it look pristine. But the opposite is definitely true: if you can't even bother making it look clean and tidy when trying to sell, I'm pretty sure you've not taken care of it in the past!
Cleaning in general is also usually a core part of maintaining something. Dirt and grime lead to wear and tear.