You have to become very good at marketing and client (not customer) service. 

You can't imagine how many customers get upset over this. 

You were given a credit, so you have their money.I would not try to collect anything for late return of keys, assuming you had access to the property and they were really out.I highly recommend you find a landlord/tenant attorney and have (pay for) a conversation about landlord tenant laws and customs in your area.  

I want to start my own firm so that I can streamline the buying process for my own investments.

My son is our real estate agent and it's his first deal ( we're his first customer!).  

As a new real estate investor, you should start in your own back yard where you have the advantage of first hand knowledge of the market, the customs, expectations, laws, and a network of friends and acquaintances who can help with advice.  

Customizing your message is going to work fine for you.This brings up an idea, perhaps BP can "certify" a Pro Account user who has been on the site for at least 12 months and has a clean record.

The answer depends on the speed you can get a simple, professional website with basic information and a fillable form to capture customer data.

Instead, think respectful and professional interactions, open and honest communication, prompt response to tenant needs, establishment of good property management systems, and customer service. 

If you are an electrician and you charge $75 an hour nobody would chastise you for turning down a job offer from a customer who wants you to work for $12 an hour.