First things I knocked out were...

I am trying to educate myself as much as possible but probably like anything you can only learn so much before without going to the school of hard knocks.

He's been doing it with CMG for 2 years now, and they've already knocked their $400k down to $40k left.

., but over the last few days she totally stopped responding to texts, calls and knocks at her door.

You wont knock it out of the park, but a solid earnings stream might be had.The other side of the coin is a poor TK provider might sell you a property that has hidden issues and a poor tenant in a sub optimal neighborhood.I don't think its one or the other, I think the vast majority comes down to  the integrity and market conditions at the time you buy.

I'm looking to start doing some direct mail marketing, driving for dollars and door knocking to stir up some off market deals.

Knocking on Doors?

It would be like door knocking, but on the phone.  

Is it bad business to show up unannounced and just knock on the door?

And knocking on doors (aka "driving for dollars") is the 1970s way of doing this.