Is the property being sold in 3 weeks op?
If it is, you need to get really drastic.
I would hire a large storage container for the front garden and take everything out that they want to keep.
This will help them 'pack' for their new home, since they will be able to see how much space they have.
Then I would pay for a house clearance.
Unless the furniture is mid century solid wood, Ercol/G Plan/Danish it will not sell in three weeks, or possibly ever.
Encourage your parents to think in terms of what they will gain by choosing the items which are either essential or sentimental.
Be careful with the sentimental stuff! I would give them a large box each and say they can pick out sentimental items to fill it with and the rest will have to go.
If you go through each room asking if each item should be kept or binned you will need at least six months. I'm not kidding.