Making it easy for merchants to accept credit cards via their smartphones, Square has quickly grown to be one of the largest mobile payments providers.

You can then either use your smartphone, or an mp3 player to connect to it.

problem with that is most of the lower end tenants do not have computers at their homes they may have smart phones but.. collecting rent in that space is a tough

You have refrigeration, Air Conditioning, a smart phone, the internet, Air Jordans, two computers, take vacations, go to college, 37 flavors of ice cream, have 365 channels of TV (I didn't say they were worth watching ;-) health insurance, life insurance, fire insurance, auto insurance, home owner's insurance, SDIRA, 401(k), plastic surgery, viagra, fast food and can call the shots of just about anything you want to do or be.I grew up in a 790 sq ft house with 4 brothers, a cousin & aunt and my folks. 

A "comp" might be on the other side of a fence from a property and not as good of a comp that's two streets away.One technique:  If you use a smart phone, download the realtor.com app  Use the "search nearby" and click on "scout" at the bottom. 

One of the reasons that poor people remain poor is that they are the greatest consumers.....my lower income renters have the $1,500 tv's and latest smart phones but they take public transportation and will rent forever.

I am getting a smartphone (I know it is about time!)

You can now communicate with the contractor and keep tabs on how things are progressing on the property all from your smartphone screen while you're at work.

You can always pretend to be delivering a package to the prospective renter and ask the neighbors about them....neighbors, especially inner city, love to talk smack.If you are renting single family units....always do a move-in video....smart phones make it easy.  

First of all I'm writing this from the new BP app on my smartphone.