Where we have several student rental properties, it is not unknown for us to get the odd high maintenance tenant - typically someone who is living on their own (outside of university residence) for the first time who comes from a privileged and sheltered upbringing  (but not always),   Rather than establish a poor rapport with a new tenant by getting into a back-and-forth over procedure, we explain the reason we ask for service requests in writing is to ensure that no details are forgotten when we review the work with our handyman or trades.   

I had her sign an agreement stating she was accepting the house as is with any additional faults known or unknown .

Realtors costs for the sale ($7800 at 6% of $130K), insurance, holding costs, utilities and electrical and have a buffer for unknowns.

Such justification might be to state that there is no intention to purchase as the optionee must qualify for financing at a future, unknown date.

That's one of the things I have found to be the bigger unknown in a flip. 

Also, just the fear of the unknown.....how could I ....out of the possible one billion plus Google users.... make a case to hopefully change the mind of the mighty Google that already knows so much about me?

The real unknown to this deal is what you are going to find when you remove the existing slab.

It's a great idea if you're a seasoned investor and doing long term holds ORif you're a seasoned flipper and just need gap funding ORif you are confident in your investment strategy.So, if the unknowns are minimum, the risk goes way down, and the suitability of borrowing from your retirement goes way up.

More wiggle room for unknowns.  

I see no way of comparing the two options, opportunity costs just can't be identified, market conditions vary and your devotion to activities is unknown. :)