But if it was me, I would document legitimate repairs or damage just like any other move out and return the balance.

Twice we have rented residential units to legitimate companies (non-profit organizations) for their workers and/or clients with no problems.

I love learning and am constantly soaking up information on investing/managing my money and while I generally like putting my money in accounts where I can let it sit and grow, I also legitimately enjoy learning about the grind that goes into real estate investing and the different ways to approach it.  

Your tenant may have a legitimate concern, BUT it could also be a ruse to set you up for a lawsuit. 

Personally, I don't see how you can legitimize moving someone out today and moving someone else in tomorrow.

I'm just wondering what are the typical documents that a legitimate company will ask for?

If they are a legitimate person, they will give it willingly. 

In fact, if you come across a broker that promises to fit you into loan for which you don't legitimately qualify, RUN!

I find having a LLC does give credibility, as does filing taxes legitimately, registering with your city etc etc4. surround yourself with credible CPAs, attorneys, escrow companies, title companies, so even if the seller doubts you, your team around you can vouch for you and instill confidence in the seller5.

If so, yes, it's a legitimate charge, although if you left the unit in really good condition and clean, I would not have charged for it.