To answer your questions here though:Alternative options - I was going to sell all my stocks and etfs, take out a HELOC, and some credit cards to try to shoestring together enough to the deal without a hard money loan. 

Even more than credit, the more experience and capital you have, the more aggressive a lender is going to be.

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

Caution:  Personal credit card financing is a huge risk and good luck finding 0 percent interest today.

@Matt Wan Many lenders require wet signatures on at least the Promissory Note but if yours does not, as a US citizen you could do the entire signing anywhere on the planet with a good internet connection + mic + camera via Remote Online Notarization (RON) for less than $200.If wet signatures are required, the answer then depends upon what foreign country you are in.

It's likely a scam anytime they sound like AI or the grammar is weird or they use the word "kindly", if they ask for you to respond to them off-platform, if they say their employer will pay for it, if they ask if the dates they've specified are still open (when they can clearly see they are open on your calendar), if they ask for a video of your property (they probably want to set up a duplicate listing with your photos and video and scam others out of money), or if they want a long term booking and they have just joined the platform and have no reviews (could have been evicted from somewhere else, and hope a newbie will let them in with no background check, they'll end up not paying or credit card is bad).  

You may find this a viable option with rates where they are, especially if your rental income continues to cover the mortgage.Home Equity Line of Credit (HELOC): If your property qualifies, you could use a HELOC to access the equity without disturbing the current mortgage.

You have to put 5% down vs 3.5% down, rates are usually higher, but if you have strong credit, your PMI cost can be cheaper than that of FHA loans. 

But the risk to the seller on this deal is immense you have a guy tying up a property who has no ability to close it and then sell it to whoever will give him money and that person could be nefarious get into title rip rents could give 2 craps about the sellers credit etc etc..

A couple developers we work with skip LIHTC and the tax credits and go the route of impact funds to help alongside other debt and sometimes equity.