we own a second home in newport RI that is 2 blocks from the harbor. sooner or later i will run short of marriott points and was thinking about using this home as an exchange fro some foreign spot

Anyone ever do this? I looked at a few sites but dont really know what is legit or not. Any experiences or advice that would get me in the right direction? Is it even a good idea?

There is a timeshare exchange company, Dial-an-Exchange, that accepts deposits of a week from a managed vacation property (somewhere to get the key, make sure that the linens are fresh when they check in, etc.) into their timeshare exchange system. While I do not have any vacation property myself apart from my timeshares, I use them myself for timeshare exchanges.
I just made an exchange through them for a 2 bedroom apartment on the Croatian coast for early August of next year. Last year, they got me a 1BR unit on the French Riviera and a canalboat on the English waterways, both in summer. THey have also gotten oceanfront exchanges for me in Puerto Rico and Grand Cayman. They often get country cottages in France, Ireland, and the UK deposited into their system. The company started in Australia, and now has offices in the UK, US, China, and South Africa. Unlike the home exchange programs, where you have to find someone else who specifically wants your house, DAE uses the timeshare principle where your deposit of your property is a credit that entitles you to take anything that is in their system. Their website is www.daelive.com

We used Homeexchange.com once and had no problems. There was quite a bit of email contact with the other couple beforehand, so we felt secure in the exchange process. Although we didn't end up using their home, due to a travel conflict on our part, they used ours and left it in perfect condition. The H-E website gives you the option of exchanging homes, or looking up vacation rentals and second homes, in case a monetary exchange is preferable. Good luck!