Marriott Vacation Club International is Marriott International's timeshare resort business unit and brand.
To learn more about Marriott Vacation Club, a good place to start is
http://www.marriott-timeshare.com/
Marriott has several resorts on Hilton Head Island and in Palm Desert. The trading strength would depend on which resort, which ownership season, and which week you book within that season and use with Interval International (II). Even then, you can only go by anecdotal evidence, because II does not publish any sort of quantified trading value information.
"5 star" and "Platinum" are two different things. "5 star" (now called "Premier Resort") refers to II's quality rating for a resort. Platinum refers to Marriott's floating usage season. For example, a resort that's rated "Premier Resort" could have a Platinum, Gold, and Silver Season.
At this time, Marriott-to-Marriott exchanges are handled through II. A few older Marriott resorts are also in RCI. A few Marriott timeshare resorts in Florida are part of a collection called the Florida Club, which allows owners to book at some other Marriott timeshare resorts in Florida. Also, Marriott Vacation Club has a points-based system for Asia-Pacific, but it's not available to U.S. residents. There have been rumors for a few years that Marriott will eventually offer a Marriott-operated alternative to II for Marriott-to-Marriott exchanges, but so far this hasn't happened.