Either I'm missing something (or just struck upon a brilliant idea), but the important point of sending you an empty box isn't that they save you a trip to the postal office (although this is a nice side effect), but that the empty box psychologicaly induces you to sell your items, and Sold takes a cut on the sale. Just as charities figured out long ago, if you send someone a dollar or even a quarter they'll donate, same goes with an empty box. It is just too easy to sell something, and besides, they sent you this nice box! I think the psychology of this strategy is genius.