You don't need to buy Disney Dollars second-hand from other people. Chances are if someone has them and isn't spending them, it's because they are collecting them. I don't think too many people just come back from WDW and "forget" about extra money.
You can buy Disney Dollars at The
Disney Store, and you can buy them at WDW. There is also an address where you can mail-order them, but I don't know the address. They cost face value. If you keep your receipt, you can "sell" the Disney Dollars back to Disney (at WDW or TDS) for regular US cash.
There is also a Disney Gift Card that is great for saving for WDW vacations, you can buy it at The Disney Store, almost anywhere at WDW that has a cash register, or even online. The card is reloadable so if you want to save a little at a time and not spend it elsewhere, you can just add money to the card at The Disney Store whenever you want to. The card can be used at WDW and DL,
DCL, and all Disney resorts, restaurants, and shops as well as at The Disney Store and online at disneyshopping.com or disneystore.com (in case you somehow don't spend it all on vacation, yeah right!). The only place you can't use the card is at a few of the small carts in WDW that are cash-only and not set up to accept any kind of cards. I'm pretty sure you can also use the money on the gift card to pay off your reservation.
