I have sold two small contracts.
You can sell them on your own....
But I sold mine using a broker (one was The Timeshare store, the second was another broker) IMHO there was no real difference between the two.
The broker will ask you got get a statement from Disney as to your current point status. The broker should also be able to advise you as to a good price point.
Once you get an offer then Disney gets involved. Disney will review the offer and determine if they are going to let it pass or not. If they do, then the deal is processed by the closing company, most folks use the one the broker recommends. If they don't then Disney buys the points. You get paid either way.