Maybe you can use that platform?    

Search engine optimization on the platform.

In the past I've just had my tenants send me money via Venmo, but I'm developing a need for a more professional platform.

Brokers have the MLS and other platforms, contracts, disclosures, risk mitigation, some convenience to the seller. 

BiggerPockets will be a great platform for you to take your journey to the next level.I highly recommend completing your BP profile to let people know where you are and what you plan on doing.Get specific with your goals, i.e.

My thought leadership platform is Youtube and Instagram. 

Do you have any websites or platforms that you recommend for me to use?

The strategy was described as to go on to craigslist and to find the rental apartment listings and message the people and tell them that you are an investor and if they ever need to sell im your guy.I'm guessing you can find and use whatever platform that contains rental listings and contact the property owners. 

I had a tendency to want to turn this into a "passion project".2) Being on top of your calendar at all times - especially if you are listing across multiple platforms.

I am sure you will find the knowledge and resources you need here on the biggerpockets platform.