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

Bottom line, lots of investors sell properties "in-house", if the investor has been around for more than a year or two and has a lot of transaction history they likely only buy the properties with easy title work and outsource the rest to title companies, so you're probably fine.  

NIce 100k house sold for 25k because of the stigma and history of it in the area

I recently purchased a land from the tax deed and when doing my research I didn't find any history of a lend or debt owned.

They used both of their credit histories and income to qualify for the financing so that's what the underwriting is based on.But you also have to look at the tax implications.

I just refinanced an investors STR, he had to supply 12 moths trailing history of the STR income. 

History is almost and completely irrelevant.

First Republic - CAFremont Bank- CAFrost Bank - TXFulton Bank, PA, NJGFA Federal Credit Union 10 yr draw, 10 yr repay, MA, NHGranite State CU, NH 90-100%Great Lakes Credit Union, Chicago area ILGTE Financial, Tampa, Sarasota area FL w 2 yrs tax history on propertyHanscom FCU, TXHSBC 70%, for premier clients only.

- Credit + Payment history.

Based on my equity, creditworthiness, and history with them?