There are several places where you can sell your points. The Tug Marketplace and Redweek are probably the best options. However, keep in mind that you won't be able to sell them for anywhere close to what you bought them for. I don't know the going rate these days, but I think it is close to $3-$5 a point. Marriott adds some heavy junk fees to the buyer.
You do need to make sure that you don't still have an outstanding mortgage on the DC points. If you do, you need to pay that off first. No one wants to buy a timeshare that still has anything owed against it.