This solution worked great for my house.  

That's a very creative solution from @Kenneth Sok; given that the tenant did not pay the recent month's rent, perhaps it is not the best solution in this case, but I could definitely see the value in talking with the tenant and exploring the issue if you think your prospective buyers might be interested.

Outside is a different story, as tree roots or shifts in the earth can crack a sewer line.If the smell is coming from the water lines, first thing to do is have the water tested for chlorine level.

Check out my creative solution. :)https://youtu.be/jaV4naZGjCM

This seems like a safe solution.

Orlando,I'm wondering if a possible solution would be to have your name added to the property deed in order to protect your interest.

I don't think you need such a massive property management solution for 18 properties...

etc etc.They have a problem and you are the solution!

Having an engineer take measurements, do a few calculations and recommend a solution will not cost that much.A local flipper renovated a 1950s bungalow not to far from my personal residence.  

What are the condition of the appliances, are there any, trees - how many and are they overhanging on the house which means I need to get my tree guy - I learned from a property I bought that had a nice yard but for living in it not renting since it's a cost to maintain, size 3/2 are good in my area and not so much 2/1 depending on the area - I live in a college town but a 2/1 in a family neighborhood isn't going to rent well, age recently for me as I lowered my range to include older, pre-1980 to 1960ish,  homes so now I have to consider electrical and plumbing, overall condition of the property.I can't speak for a fix and flip but I would suspect they may look at the property for "some" of the same things - maybe not.