We have only bought, not sold, our timeshares. If in future, we choose to sell, we'll likely go through Redweek (or similar) or a trusted broker. Trusted brokers take maybe a 10-15% commission, which is far less than the huge 50% Marriott takes. Both generally help find a buyer and assist with orchestration of the sales process to a buyer.
My guess is that MVC owners, like the Cissy and others, choose Marriott Resales because they are a known trusted entity. Why not sell back to the person you likely bought from? Marriott can also provide a simple E2E transaction with little effort from the owner. A huge plus for those who don't know otherwise or don't have/want to spend the time. They just want out to stop the MFees.
Sorta like when you buy a new car from the dealership next door. They will buy your old car on the spot for less money than it's worth, but with no hassle. The owner gets rid of the old car, no need to find buyer, go through negotiations, multiple showings, etc....and dealership gets a good price for the used car and sells you a new one. Convenience has a price for many.
For me though, I'd rather put in a little bit of work...and save 40% on commission. If you own an excellent resort, week and season...it shouldn't be hard to find a buyer. OceanPoint 2BR OF is a great week...especially if Plat season (?).