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. 

There is another way around the deposit that can bring to the closing, if you get the seller to agree to a second mortgage on the property with a negotiated interest rate favorable to the owner of the property, depending on the flexibility of the mortgage company and it sounds as if the mortgage company is working with a flexible lender as he is recommending you take a loan for the 20% deposit, the actual lender he is working with might accept the second mortgage as the deposit and close with you, this no money down technique works best (gets the owners on board) if you offer a decent rate on the 20%, additionally if they are in need of the funds soon but want to sell you can make the terms a very short term as I offer when I want to buy a property with this technique, offer a two year balloon payment to start, 24 months is a long time to refinance the property and now you enter my zone of leveraging (Mwwaahhahahahaha) you do a few upgrades that will improve the properties value (creative equity) I have done insane cash out loans this way (I purchased a home for $10,000 put $5,000 and the appraisal came back at 110,000, I took a 90% cash out loan and pocketed $96,000 check in my hand, remember that the property only cost me $15,000 in total, sooo....

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.