First and most importantly, use a payment method that you have confidence in. I use my American Express card for almost all my online purchases, because they have a 20+ year track record with me of outstanding customer service. If something goes wrong and I call them, I can get my money back.

Next is to buy from a trusted seller. Amazon has consistently treated me right. I recently bought _Desk Ref_ in hardcover from them.

If what you want isn't from a seller that you trust, see if you can't buy it through Amazon Marketplace or eBay. Both companies, in my experience, will support buyers when sellers don't deliver.

When things do go wrong, be polite but also be persistent. I like to give the seller five working days to get back to me before I give up and file a dispute. A lot of times, less scrupulous sellers will offer you a partial refund rather than a full one for a broken or missing order. Stand your ground and insist on having the problem solved to your satisfaction. Just as with brick-and-mortar companies, some will treat you right and some won't.