I bought three weeks in secondary market and am having excellent results with exchanges via Interval International.
I traded Grande Vista Platinum for Desert Springs, traded Thanksgiving 2011 Canyon Villas for Feb 2012 so I get two consecutive weeks, And I traded a Feb 2012 (the last purchase came with a week) for Spain Playa De Andaluza.
It is important to reassure buyers that secondary weeks are still good to trade. The big pressure from Marriott sales rep is that you must by points or else you will be stuck with a week that cannot reserve or trade.
Remember enough points for a platinum week cost $25,000 to $35,000 and a secondary market platinum might be $2,000 (Manor Club) to $10,000. I paid $7,000 each for Canyon Villas Platinum.
Do not let your friend get pressured into points.