I purchased my first investment property through an online auction for delinquent property tax.

What documents do they require, what credit scores do they allow, how do they verify previous rental history, etc.?

Never put all your eggs in one basketread the PPM and documents and have an attorney review understand what your investing in, fees and hold Timeunderatand their track record a lot more to consider those are just a few 

@Andrew Moresi I have been looking around, I found a couple promising properties online and am also in contact with a realtor.

If you own several apartment buildings your best interest might be to speak with a asset protection attorney versus asking questions online of just anyone who might respond.Good Luck!

Quote from @Obed Calixte: There are a ton of free tenant applications online and through property management software companies such as turbotenant, rentspree, zillow etc.You will want your application to meet any state specific guidelines (if applicable). 

I do SMS, cold call, mail, online, wholesalers, etc.

Not one single example, yet everyone pushes it like it's the ultimate protection policy.As stated above, the best protection is to follow the law, treat your tenants honestly, and document everything. 

Some will let you rent your primary and still do a DSCR loan on the subject, however, this would come with the added scrutiny of really documenting and evidencing you have no intention of moving in to the subject.  

Trust documents are not required to be publicly recorded, ensuring that details about the trust remain private.2.