If you bought on eBay, you should use an escrow company, and the escrow company will create something that works as a contract (joint escrow instructions describing the price, unit, and terms at the minimum).
That said, you may want to get an ROFR waiver before starting with the escrow company (if Marriott exercises ROFR, you would be out the escrow fees). I have no direct experience, but all Marriott should care about is the unit type (Waiohai 2bd. annual ocean view, etc.) and price. I would try getting that via e-mail; Marriott may issue the waiver conditional on the deal closing on those terms. Otherwise you will probably need to write a formal contract or an open escrow.