Marriott to Marriott trades via II are pretty easy for just about anything you want so long as you are giving up your entire 2 bedroom for premium trades. For Platinum to Gold season, you can usually get away with just giving up the one bedroom side.
HOWEVER, ski weeks and Carolina summer weeks are as close to impossible trades as they come, even for Marriott owners. If you want a ski week or a summer beach week in the Carolinas, there is no way around having to buy one of those weeks. Normally, you can get away with buying a cheap trader and exchanging into the more in demand weeks/properties, but not ski weeks and not summer Carolina weeks. If you only want a one bedroom or a studio ski week and are willing to give-up your entire 2 bedroom Grande Vista (if that is what you buy) then you should be pretty successful, so long as you don't want a holiday week. All the ski resorts lock off and you can bet that those owners who deposit into II are locking off their unit, so there are not many full 2 bedroom deposits.
Bottom line is that any platinum 2 bedroom Marriott will trade very powerfully in II, especially for other Marriott properties. But, ski weeks and Carolina summer weeks are nearly impossible, even for owners of other Marriott ski weeks and summer beach weeks. II works mostly on a like for like exchange principle. For example, say you want to exchange into a two bedroom Mountain Side in February. First in line for those trades will be other Mountain Side owners Platinum Plus and Platinum week owners. Next will be other Platinum Marriott ski week owners from the other resorts. Some Gold week owners at the ski resorts could be next in line, depending on the trade power of their weeks. Afrer all the other "like" ski resort owners will be other Platinum Marriott owners giving up like size units. It would be reasonable to assume Platinum beach owners would be first in line over any of the inland resorts (Orlando, Branson, Williamsburg, the desert), since platinum beach weeks are in such high demand, but I'm not sure. What I am sure about is that platinum 2 bedroom deposits will beat platinum 1 bedroom deposits in terms of who gets the trade first, though.
So, best option is to buy a week you would actually want to use most of the time. I know that is not an option for many people, including myself when I first bought into Marriott a couple years ago. The next best option is to buy a high power trader (any Platinum Marriott week will do) but know that you will be disapointed most of the time, and maybe even all of the time if you expect to get a 2 bedroom ski week or summer beach week in the Carolinas.
Hope that helps, and good luck. I will just add that Marriott to Marriott trades via II are getting better. Last year, the DC really took the best inventory from II and used it for DC members. This is still happening a lot in II, but it has gotten a little better. Marriott takes the best deposits from II (i.e. ski weeks summer beach weeks) and gives those weeks to DC members. Marriott is supposed to replace any week they take from II with a like week but we all know that "like" is very subjective -- same as a platinum inland Marriott week being "like" a platinum ski week in terms of a trade.
what is the tiering for exchanges? let's say i own a platinum 2bdrm in Grande Vista summer platinum and want the same sized winter week in Utah- how does the process work? Do points owners get preferrence via Marriott before it gets to II? If there is no points owner who put in a request for that specific week on file, does it then go to II who holds it for x # of days (24?) in case a Marriott owner wants it? If not, does it then go into II's general public pool?