Your book sold! The buyer paid.The sale is not complete until the buyer has received the book.  I can't number the books I've received shoved into plain manilla envelopes and bounced across the country. Once, I even got antiquarian books inside a cereal box!   Even in a bubble mailer, the book, or worse yet, books, can be damaged by sliding around inside the envelope. If you want a happy customer, package the product adequately.

   Wrap the book in some sort of paper or foam or buble wrap (not newpaper - it bleeds newsprint). Make sure spine and corners have enough padding to prevent bumping. Use an envelope that leaves only a small amount of excess, if any at all. Ideally, a box should be used, but who has the right size on hand? If you have multiple books to ship, wrap them individually to prevent them from rubbing each other.

   When the buyer opens his package, the book he saw in the photo is the book he should be receiving. And when it is - you have a satisfied customer and a successfully completed sale.