While we have rented direct - but there is a greater inherent risk in doing so...you need to find a trustworthy owner that is not going to stick it to you. We found one here on the DIS, and used references to ensure the person was legit, but we knew there was risk in doing this...I probably wouldn't rent direct from an owner again unless I knew the owner.
With the reputable rental companies there is almost no risk - the companies guarantee your reservation or you get your money back. (The "risk" being that something happens with the owner of the points - the rental company will guarantee your money but I have heard of two incidents where the owner of the points weren't paying their dues, and Disney pulled the reservation...the rental company reimbursed the renter, but of course the reservation was lost. This is a again 2 incidents against 1,000s of rentals, so I would consider the risk very low.) Generally, you will pay about 1/2 to 2/3rds what Disney charges for a room by renting.
Another negative to renting - you can't make changes to your reservation...if you think you will need to change your dates - don't bother renting.
As others have pointed out, for a September trip you are already less than 7 months out from your dates. At this point, owners can book anywhere, so many of the more popular resorts (Boardwalk, Grand Floridian, Beach Club, Poly) will already be booked up. You may still be able to get Animal Kingdom if you book quickly. The good news is that by using < 7 months points you would likely save some money versus priority (> 7 month) points. Contact one of the rental companies and do a price comparison with Disney.