As in any area of real estate investing, it's best to know your niche and the areas you're wanting to do business in.

However, if you're a W2 high income earner or business owner, I can't imagine it makes any sense to flip houses, mail mass post cards, wholesale, or build lead gen websites with the value of your time being better spent other places (such as learning about real estate and increasing your income through proven means), such as at your 9-5pm or growing your business, and your real estate is a long term process and you don't need "home run" deals to qualify or make it work.

If you think things are busy now, wait until you get stuck with a problematic tenant!

Can you afford to pay someone $40k per year to manage part of the business?

Quote from @Terrence M Brannon: Quote from @Garrett Cuppels: spent $15 on a book about Airbnb rentalSkip the Flip by Hayden Crabtree which was freeHow To Start A Profitable Airbnb Business in 4 Weeks: Optimize Your Profits and Go From Side Hustle to Passive Income With Minimal Effort What book did you buy? 

You have to take your pick between charity and business.

I bought a business where the prior owner had it in an S-Corp, the S-Corp and he was sued personally for three million.

One thing in real estate and probably most other businesses is that most people including me don't know how to measure and evaluate risks.  

In my experience, the good ones are expensive, typically require a subscription, and sometimes they even do a physical inspection of your premises to verify it is a legitimate person / business in US.

You can't use a PO Box for a business entity.