Competition keeps people on their toes and prevents laziness.

Contractor fees may be higher in an area on average and prevent meaningful returns vs flipping.Cleared lots might be way too expensive in an area worth building and kill the deal.I mean,  I've built two houses and I won't build unless the appraised value will be 30-40% more than what I built it for.  

Russell Gronsky WBS is useful those who understand, however for those who don't understand it and they implement such a thing. some of the contractors who is investor grade, can't even issue a lien waiver.

Then implement even crazier rent control in SF (or god forbid, make condos subject to!)

It is probably set backs that would prevent it so first stop is the building department and they may even have something you can look at online first. 

Have there been previous insurance claims against the property that will prevent future claims?

@Brent Bialik So, I do think realtors do help with both the negotiation and 'prevent hurt feelings' aspect of it. 

Huy Thai while its possible there is a scam there my bet is that it’s likely investors trying to implement some kind of seller finance strategy.

You will never prevent this stuff, you can only reduce it.

what course do you take to get started and really start implementing the idea of go out and finding deals.