For buying points long term, be sure to check first how many years it will be before the points expire. Also, be sure you are buying Disney Vacation Club (if that is what you really want) as opposed to buying points at a non-Disney timeshare with booking privileges at DVC resorts.
Buying points is different from renting a reservation from a member or a broker. If you want to buy points from another member, you'll need to go through an attorney who specializes in timeshare resales. DVC will need to review your purchase because they have Right of First Refusal. If they deem the price per point too low, they can purchase that contract right away from you. That could take up to 90 days to complete the sale.
If you are looking for a single trip, you want to rent a reservation. You can use the Rent/Trade Board on the DIS (under DVC boards and you must be logged in to see it), you can purchase a reservation from another site or you can use a broker who matches up owners with buyers. Never respond to private messages from people offering you points to rent or existing reservations. That is how people get scammed.