Demo racquets for sale are in a separate category if you scroll down the Used Tennis Racquets page.
The used racquets are all racquets previously owned by somebody else and decided to trade in their racquets to TW.
When buying a used racquet from TW, you can return the item back within 30 days.
All racquets purchased as Used (including demos) can be returned within 30 days of the invoice date for a full refund (excluding string).
Direct quote from the TW website.