Right. The details can be unique for each trade offer, but this essentially means the gift card is being offered at 50% of its face value.

This could happen if the trader has an inventory of cards that are about to expire, or possibly where the cards have legitimacy concerns (e.g., could have been obtained via fraud where it's only a matter of time before the issuer discovers the fraud and cancels the cards), etc., or they could be legit but just for a brand or product that has very little market interest.