The only time I saw black residue on walls was from a flip I purchased with a not venting correctly oil furnace.

A city with a diverse economy might have:1) A big college/university2) Mining/Oil/Natural Resources3) Tech companies moving there4) Big businesses like Fedex, Nike, UPS, Amazon, Samsung, or LG opening up shop there 3) An automobile plantTHAT is the difference between a one horse down and an economically strong and diverse market.Make sure that the market you choose is NOT supported by just 1-2 economic sectors.Pillar #3 - Population Size/GrowthWhen I first started investing, I bought a few houses in Birmingham, Alabama.

Most of Texas is an oil based economy, even if there is not a well within 100 miles of you.

This business is full of snake oil salesman, piranhas, and know it all's.

Although the crude oil exploration has leveled off a little lately that still is a good investment.

As I said previously you're obviously selling snake oil and have nothing to back up your wild claims so good luck in your endeavors, but people like you should be the last people giving newbies advice. 

Before the recent boom even oil companies rarely had a 5% profit from gross sales. 

Remember that "All that glitters is not  gold", "Luck favors the prepared individual" & "There are more than just a few snake oil salespersons and also good folks in the RE and finance world." :-)

You may add a little dish soap to the water mixture to cut any dirt and oil on the surface you are cleaning that may hold mold.

I know how to change the oil in my car, but I pay someone to do it because I want the service.