Something that's sold has been exchanged for money. When new neighbors buy the house across the street, you'll see a sign appear in its front yard that says "Sold."
Once you sell something, you can describe it as sold, whether it's a sold item at your yard sale, waiting beside the driveway for its buyer to pick it up, or the sold trees at the Christmas tree farm, marked with colored tape. The word sold comes from sell and its root meaning of "offer up or deliver." Offer up the highest bid on a painting at an auction and you'll hear the auctioneer call out, "Sold!"