Yes: a gift is an item in your inventory within Steam; a key is the letter + number combination you input to "Activate A Product On Steam". Gifts can be sent by email or to a Steam username, keys can be shared via any text-based communication.
You do understand that you will not get credits until winner gets his won auction? If the winner won't get his winning Listia will return his credits back from "credit pool" and you will get nothing.