Tug is a good suggestion. Redweek is another timeshare site a lot of people use. So is eBay. All of those involve handling the transaction directly yourself.
If you want to use a broker, there are a number of those who connect owners and renters.
Some brokers specialize in particular resorts or destinations, so you might have to search around more. For example, there is one who specializes in DVC rentals, there are some on this board who seem to specialize in Wyndham Bonnet Creek, and others who handle other areas near WDW.