In Hawaii our contract specifies if the property is to be delivered free of all personal property, trash, etc. I've seen situations (fortunately none of my transactions) where the Seller was charged a clean-out fee because they left the things they didn't want to move. I like to set final walk through at least 3 days in advance of closing so money can be held in escrow in case there's a problem.

I haven't had this happen, but I guess I would do what it would take for the house to pass the final walk thru. I'm assuming no personal items were left behind. I wouldn't do anything with those. Depending on my relationship with the seller, I would probably also give them a call and suggest a person that could be hired to clean up their mess. 

I try not to.

I prefer not to bother them when they are moving - except that I may offer them to have lunch with me on that day.

I had to do it once as the sellers had moved to China. 

Many times sellers have left a mess and usually it is mot my listing as i go over this with me sellers before what is acceptable and what is not. I was just a buyer agent and upon our walk through we found a mess. When asked what she would do teh listiong agent laughed, the buyer refused to close and i started picking up boxes. The buyer seeing me do it joined it, the listing agent showed up and leaned against the wall playing on her phone. I wrote about it today in my blog. The seller got an eye opening at closing

My guideline is that the seller should leave the home how they'd like to receive it.  To most sellers, that means clear of their belongings and trash, and then professionally cleaned.  We discuss it to make sure we're on the same page.

An email to the seller so you have a written response would be a good thing to have.  The contract is between buyer and seller, but as agents we do things to help our clients.  It's much less rewarding to do a nice thing when the client ditches you and leaves you with the mess.

The "Free" category on Craigslist can get a lot of stuff removed pretty quickly.

The seller should not have left you to do what was their responsibility.

I've never had this happen. I probably would have had a discussion with sellers up-front as to how the final clean out would go if there weren't there to supervise. But as to liability, no idea.

No.