But outside of that, I don't think a stabbing in the unit is going to prevent too many people - other than the person that got stabbed obviously - from renting there given that the person didn't even die.  

@L'aura BradfordBanks don't like it & further, go out of their way to prevent a buyer to tie up a property & then assign the contract to someone else. 

Anyway....I use these plumbers for the rapid response, clogged toilet, dripping faucet, water line break jobs.  

How could one prevent a stolen deal?

That way they will likely keep the property up and pay, but if they don't they will be reasonable with a plan to move them out to prevent negative history on their background while you can get possession of the property back quickly to re-rent it.

Is there anything preventing me from starting to market the unit for rent while he still lives there?

In return provide them with reliable people in your network so it's win-winTip #5: Get a Google Voice Number - Trust me you need this especially if you are posting ads for RentalTip #6: Preventative maintenance - MustTip #7: Be proactive and not reactive to your tenant's need.

For instance she would tell me we needed to send someone out to snake the main drain line of the house because it was clogged.

To prevent that from occurring, it has been suggested that maybe we Realtor/Investors should have the sellers sign something that states that they do indeed know that we are going to flip this thing for a profit.

But, don't let you're fear prevent you from learning about other profitable parts of the business.