Quote from @Isaac Watson: @Charles Perkins are these meetups on platforms like meetup.com?

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Has anyone bought from the platform?

Platforms like Zillow or Redfin can give you insight into comparable properties, and PropStream is great for deeper analysis like liens or ownership history. 

BiggerPockets is an excellent platform for networking and establishing connections.

Some custodians may offer access to platforms with investment opportunities, as well.

Do you know of any websites or platforms where I could go to analyze and practice seller finance and creative finance deals?

You're right there that's not a single platform that does everything but I feel like Baselane comes close to it.

What this stems from is that the IRS requires that a taxpayer trace interest expense to the nature of how the proceeds are used. 

My top priority is having maximum flexibility with what I can invest in within my HSA, second priority is ease of use and convenience of the investing platform, third is minimizing any account, trading, or management fees charged by the HSA provider.