I so know how this feels! Having done this, and moved several times, I know how overwhelming it might seem. Personally, since we were moving into my late mom's house, I removed all of their things and put them into storage. Then, once our things were in it was easier to figure out what to give, sell or keep.
First thing to remember is, a house should be staged properly to sell. This means you do need to declutter. They suggest keeping furniture but removing mementos. Also, only two items on the top of any table or piece of furniture.
Secondly, second-hand furniture doesn't really sell well. We donated lots to our local women's shelter, as well as the Sally Ann.
Clothing should be washed. Check pockets, though! My parents had these little measuring tapes, from when my dad was working as an architectural consultant. Now, I wound it relatively easy to pack up mom's stuff, but then when my father died months later, I packaged up the stuff and my dear husband took it to the Sally Ann for me.
I took my mom's things to a different town. She had some lovely outfits as she worked until she was 66 years old, wool suits, etc., and I know they would be good clothes for someone.
I made a checklist, and motored on through this. I was really numb doing mom's things.
If you are in doubt about anything, simply keep it. Wrap it up properly and save it. My late mother adored Christmas and had THREE trunks full of decorations. I could not face going through it. I gave it away for the church to sell. I didn't realize my daughter wanted some of these things. HUGE mistake. I should have taken more time.
This article I wrote might help, it breaks it down for when you are moving them into a long-term care, but it might twig some ideas for you:
Bearing in mind that you need to keep all the legal documents for 7 years, as well as filing the tax returns.
Also, this article:
All the best. You can do it!