Fwiw, the NTHC principal is a TUG member and trustworthy. I know her name, but I don't feel that it's my place to just print it here for all the world to see.
I believe NTHC is sometimes a "middle man/woman" renting for others (for a fee), as well as being the actual owner(s) of some weeks being offered for rent.
Whether you find NTHC deposit / payment terms acceptable is up to you; $100 down
may be regarded by NTHC as showing insufficient "commitment" on your part.
You can perhaps
attempt to negotiate the payment terms to see if there may be some mutually agreeable middle ground --- 100% payment before providing
any form of contract or reservation confirmation / documentation at all does seem a bit "strict" to me, but proximity of the actual rental date may be a factor too .