As a landlord, the more properties you acquire, the more cash you will have to deal with and store. 

Purchased a duplex in Colorado and I'd like to know how long we are required to store/hold onto the tenants property they left behind after they move out? 

If you ask them for proof, they run to a doctor or a web site and get a letter.You have to determine your comfort level and how you want to proceed.

We are both in agreement that our biggest asset is the equity we have stored in these houses-- about $400k.

As an example, I personally do not care for oil heat:  I do not like the liability of storing several hundred or thousand gallons of fuel oil (effectively diesel) on the property and, do not trust tenants to keep the tank filled.Regardless of the heating solution you choose, your best return comes from the fuel you do not use (Negawatts or Negabarrels).  

They are also a retail store (the only retail in the building) and they have been putting goods for sale out on the sidewalk during the day.I don’t like it, but they are a long term tenant and I don’t want their first impression of me to be a jerk trying to throw my weight around...How should I approach this issue?

Pennsylvania eviction processYou should bookmark that web site and buy their book, "Every Landlord's Legal Guide."

If my guys and gals I fund want to use them I don't care I just don't want to deal with them.PS I like the car business if I was not doing this I would own a dealership... and I would finance sub prime auto debt.. big money in that these days.a few of my fishing buddies own big stores like Toyata and Chevey stores they as you well know make bank.. 

https://www.biggerpockets.com/store/flipping-physical

This past week, the basement became partially flooded, damaging family stuff I had stored down there and building products we had left over.