If you want to buy resale, search DVC resale. It’s pretty easy to figure out the ones that are trustworthy. They will have the easiest websites to navigate. There is one who has been a longtime contributor to the DIS. You’ll see the see the advertising on the site.
If you put yourself in the shoes of the seller, they want a broker who will give them a wide reach of potential buyers. There are probably 4-5 brokers that meet this criteria. I would be wary of contacting an individual owner through eBay or Craigslist. The possibility of fraud goes way up, and they may not know the nuances of the ROFR process.
It sounds like there is no downside for the buyer to go through a broker vs. a private seller. The buyer can still potentially negotiate the same price through a broker as they could with a private seller since in either case Disney can swoop in and snatch it if the price is too low. The only upside would be for the seller to potentially earn a bit more money for the sale without a broker taking a cut.