I know some owners and am familiar with the successes and frustrations they have had trying to rent their weeks.
A major complaint of owners, even those planning to use the time themselves, is the chronic lack of availability of desired nights and weeks unless you make a resevation way in advance, preferably one year ahead of the desired check-in date. Call at 9:00 a.m. sharp!
By all means choose a very desirable week such as Christmas week, and most any week in July or the first half of August. As already suggested, state in your ad that other dates may be available, and offer to check for availability of other dates. But do not cancel the desirable week until you receive at least a 50% non-refundable deposit. Most people who are successful in renting the week, usually receive several hundred dollars less than the maintenance fees they paid for the year

but it's better than nothing.
Do not place an "open-ended" ad with no dates reserved because by the time you get an inquiry, you probably won't be able to get the dates.
You should consider placing a free ad here at the TUG Marketplace, and also at
www.redweek.com . Some people have also had success with free ads on Craig's list but be careful there because it is rife with scam artists.
I'd recommend that you join the Yahoo Manhattan Club owners group--it's free:
http://groups.yahoo.com/group/manhattanclubowners/
Once in awhile an owner will post a message that he is looking for an extra week to rent. They also discuss many complaints about the ever-increasing maintenance fees and deterioration in the upkeep of the units, and especially the difficulty encountered in trying to book their time. Last summer they mounted a telephone and Email campaign trying to get the NY Attorney General's Office to investigate the problems. I'm not sure what the current status is.
Manhattan Club weeks are not selling well, even at rock bottom prices on Ebay. There are hefty closing fees too if a buyer is found. Most owners feel it's better to hold onto their week instead of accepting pennies on the dollar unless they desparately need the money, or relief from the high maintenance fees.
To get an idea of what your type of week package is selling for, you can contact Seth Noch, a broker who specializes in Manhattan Club and other high end timeshare properties: (212) 677-7636
www.sellingtimeshares.net
Good luck!