To not trust contractors even if family ( I was burned on multiple occasions).

In looking at many houses on their site prior to bidding I found houses with split foundations, condemned notices posted on the house by the county, burned houses, lots of really bad houses, and they are silent about the problems they have to know about! 

You have to decide on a sales price, when ownership changes hands, who pays what taxes, what if they run the business into the ground, what is the place burns down, what if someone gets injured and their is a lawsuit, what if the owners pass away or decide it's too big of a project for them.

Purchase price: $1,500 Cash invested: $1,500 This was a SFH that burned down.

An arsonist burns down your units before you insure them.

Have you been burned before?

Long story short, the fence/barn/chicken coupe/driveway/basketball court etc, remains until it falls/burns etc.

A. it's a waste your time & B. you're probably gonna screw it all up & burn yourself some bridges that could have been very important down the road. 

Never base your numbers on the expectation of that because you will almost certainly get burned. 

Renting to someone based solely on personal knowledge of them is a recipe for disaster and I've seen many Landlords get burned by coworkers, best friends, and family members.