It can be technical but underwriting is generally based on strength of borrower credit, assets/reserves, income of the property and aforementioned LTV.

@Joshua FlammIf money is a constraint you borrow against your free and clear property.  

In most (but not all) cases, lenders will use the lower score of the two borrowers.

Many of my potential borrowers have trouble meeting this requirement, 

Ive done seller financing a few times as a lender, and the borrower.

There is no equity and he is borrowing money from these folks.

I would not cash out a retirement account.I would take your STR income and use the profits to pay down the money you borrowed for the STR downpayment.Leverage low interest Stafford loans for college.

This goes against the general concensus, but I'm not a big fan of borrowing money to borrow money.

And seeing it was on a no interest credit card I think of it as just borrowing money, not going into bad debt. 

Deandre,As a Banker I am not a big fan of borrowing money for the down payment unless its from family or a close friend.