
Stick to sites with a secure checkout (look for "https" and a lock icon) and pay by credit card rather than debit when possible — it gives you stronger dispute protection if something goes wrong.
Be wary of deals that seem far below every other retailer's price, and double-check a seller's reviews before buying somewhere unfamiliar. Keep packages out of sight from the street once delivered, or arrange for delivery to a location where someone's home.
A few minutes of caution avoids the vast majority of holiday shopping headaches.