From a buyer's perspective, the big difference would be that a resale broker with experience selling
DVC contracts will know exactly what to do at every step in the process to ensure a smooth transaction. Plus, if you're buying from someone you have never met, it is good to have someone in the middle handling the transfer of funds between parties.
From the seller's perspective, the seller will owe a commission to the resale broker resulting in a loss of 10%-15% of the sales price. You might think that this could mean a slightly lower price for the buyer, but because of ROFR if the price drops below Disney's target range for that resort, Disney will buy the contract and the original buyer will lose out.
So as a buyer, I can't really see any advantage to buying privately vs. through a resale broker, but perhaps some other DVC owners will weigh in here with a different perspective.