I've rented from both points brokers and private owners. A points broker basically acts as an intermediary between you and the owner taking care of all the messy stuff like ensuring that the owner is legit and talking to the owner about your Magical Express, dining plan, and other issues. You'll never talk to ther owner and the broker never talks to Disney--all the actual arrangements are made through the owner and DVC member services. Because it's a business, the owner has an incentive to get back to you right away if you need something; I know people who've rented directly from owners who've had issues getting things done and since you can't deal with DVC member services directly, you are kind of a the mercy of the person you rent from. For that convenience, you pay an additional $3 (or so) more per point than average. It's great if you don't want to deal with an owner directly.
The service points brokers offer is essentially a gray area for Disney, which has taken steps to keep professional renters from offering this type of service. Disney doesn't like points brokers, but since the broker isn't involved in the transaction with Disney, Disney can't do anything about it. I doubt Disney will do anything to stop brokers, even if they could, because allowing members to freely use their points is an important benefit.
The biggest benefit of using a broker is that you run almost no risk of showing up at Disney World and not having a room. Brokers want return business and good word of mouth, so they make sure their owners are legit and if anything does go wrong, they'll make it better. Your recourse using an owner directly is limited and certainly not going to solve itself easily if you show up and find out an owner has cancelled your reservation. You almost never hear about this happening, of course. Most rentals go really smoothly, but that's not to say that it can't be a little scary.
Now, having said this, I don't like using brokers simply because the higher price they charge means it isn't worth it to me. I don't need someone to make my ADRs and I can easily check to see if the owner I'm renting from is legit. Also, I've been renting points long enough that I have a couple of owners I regularly go to and if they can't help me, they can put me in touch with someone who can.
Beyond the higher cost, the biggest reason I don't go through a broker is that most of them offer no more protection in the event of cancelation than the average owner. In fact, they probably offer less, as I've had to cancel points rentals before and had owners work with me and be really nice about it. With a broker, you're pretty much out of luck unless you buy private insurance. The benefit of renting points is that you pay less, but you incur some risk. That's what you have to balance out. To me, that risk is worth it at the usual cost of $10 per point, but when you weigh the risk of cancellation against the cost of a higher points rental, it's less worth it to me. I've done the numbers, and at $13 a point, you'll only save a few hundred (depending on the time of year and available discount) over the discounted cost from Disney. At that price, I'd rather just pay the extra money and go through Disney because their cancellation policies are pretty friendly.
This isn't to say that I think brokers are bad, just to offer a different perspective. David and other brokers out there are totally legit and offer a good service to both owners and renters, it's just more service than I personally need or am willing to pay for.
