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

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

I tried some solutions and steam approach around painted area to scrub based on online searches but did not help completely.

The answers to most of your questions can be found just by searching in BP or online. 

They are legit - USA Today had a pretty great article about them online.

I've asked this seller for any documentation he had and he's yet to provide anything.

Have proof on hand (easy enough to find) online of the missed tax payments and any documentation that supports that he specifically didn't pay his taxes.Probably a simply mistake on his part, but now if he wants to develop on it, he either has to land lease it from you or buy it back.I'm not sure how the whole tax deed thing works, so that would be something you have to figure out between the two of you.

They will handle the collection and distribution of payments, amortization, escrow accounts, and any tax documentation. 

I came across this article from Kellogg Insight showing how online sports betting is eating into people’s savings.

So make sure if you also want to collect last month's rent up front to not get burned, document that!