The drain cleaner snakes or jets the line and rarely are they able to see the items flowing past.Also in some cases it's a combination, it could be an old cast iron pipe with a rough inside surface full of rust, which started to act like velcro to things that flows past, or a slight settlement causing a belly, which again impedes the flow, then the tenant pours down grease, pasta and bean, well, you get the idea.Most of the time you end up eating the cost especially if the plumber can't tell you anything.Now, you mentioned that the clog caused water to back up to a vacant unit next door. 

Consider your obligation to the end buyer when the due on sale issue surfaces, if not paid off by someone in the deal you are in default to your buyer.You can not simply create a deed-in-lieu-of-foreclosure (DIL) or hold one and go down and file it and mail a letter to the seller note holder saying "We're out of the deal, we have you the property back"!

I would rather go and touch up paint here and there, which you'll have to do anyways rather than scrub paint off my new finished surfaces.

Doing research right now, but I've still only barely scratched the surface of this.

This all goes much deeper than the three inches below the surface where inexperienced real estate operators/investors frolic along with W.

You would have to sand the top surface to rough it up then skim coat with this product they sell at Lowes for like $10.

also consider whether you need a full repaving (remove what’s there, level the surface and re-pave), just a 1” or 2” layer over what’s there, or maybe just a seal coating.

Looking at the same surface level information (which I have presented here regarding my own situation - it's all been surface level, like an iceberg if you will), there is not usually a solid consensus for one side or the other.

Wipe surface with TSP (1/4 handfull in 1 gallon bucket 3/4 filled water) using cotton rags (real cotton, not the non-absorbent stretchy stuff.)Dab latex (paintable) caulk or toothpaste into nail holes with finger (very tiny amount to just slightly fill the hole) wipe with dry finger, palm of hand or rag.When dry if there's still a sheen when viewed at angle then lightly "scuff-up" surface with 400 or 600 grit sandpaper wrapped around a 1x3x6" sanding block in long vertical sweeping motions.

What are common reasons a deal that looks good on the surface would sit on the market for so long?