I've scoped the sewer line on a fewer older properties that I've had under contract. 

You should start the eviction process because she is a danger to others.

It turns out to be a 0% interest loan, but you can fudge around with that by offering more or less cash up front or more or fewer or larger installments.

seal up all holes and trappoison works well but is a danger to pets and wildlife

For commercial, shorter term notes can be dangerous. 

Fewer take the action.

Everyone can relate to stacks of money, fewer than that, can relate to 5% interest. * Costs associated with selling homes.

BUT the danger with that thinking is that they go into bad areas (D & F) because those are the ones where you can get double digit CoC but it's very hard to manage and those double digit CoCs get eroded pretty quickly with unexpected capital improvements.Lastly, being overly focused on CoC can also become costly in terms of opportunity costs.

I just want to ask what are the dangers of building a rental property portfolio through financing compared to buying them cash and owning them free and clear?

They theorized a reflection maybe (we have dangerously low humidity right now).