Using WM for anything less than a week is not possible in RCI, unless you buy retail and have access to RCI points. So, using your ebay WM contract for any kind of short stay in RCI is not an option.
Using II, a shortstay may be an option, but don’t get your hopes up for a name brand resort, especially during anything less than way off season. The choices are very limited for breaking up a week in II. It is an option, but you have to be more than flexible and lucky, and almost 100% of the time, I think you will be disappointed. Now, if you are okay with mud weeks (those spring weeks in the mountains when you can’t ski or hike) or winter weeks in say Williamsburg or Virginia Beach, then you may be very happy with short stays in II.
About WM and how it works in II or RCI for full week exchanges, RCI is the system to join if you want to actually be matched with a resort via an ongoing request. I get matches in RCI for ongoing searches to Myrtle Beach in the summer, DVC, and the other top resorts in RCI all the time. In Interval, I can see great exchanges in the on-line inventory but I have never once (in nearly two years) been matched with a resort via an ongoing request. Interval has very restrictive preference periods for say Marriott and Starwood owners. It is really tough, probably even impossible to get a prime exchange via an ongoing request in Interval. In RCI, you will be successful a lot at getting a prime exchange via an ongoing request. WM owners do best in Interval during flex time and instant on-line matches.
Hope that helps. WM is GREAT! There is nothing better to own for getting the best of the best RCI exchanges. For Interval exchanges, you really have to watch on-line inventory like a hawk and be able to book something yourself – and you will never get a summer beach week at the Marriott’s in the Carolinas or a 2 bedroom in one of the Marriott’s or Hyatt’s or other top brand resorts during ski season via an ongoing request. Hawaii is also out of the question for the top brands in Interval for anything other than flex time or lucking out and finding something yourself in the on-line inventory.
In other words, you can basically ask for any exchange you want in RCI (within reason) and wait a couple months and get exactly what you want. In Interval, almost all the top trades are out of question and you can never count on Interval to find an exchange for you, people who do best in Interval do so by finding their own exchanges. Been my experience, at least.