Cyber Monday is in full swing, and retailers like Amazon, Walmart, Target and Costco are offering alluring discounts on major brands we know and love. So today’s a great time to buy what’s on your wish list or get your holiday shopping done. But these sales won’t last long — as a reporter who’s covered Cyber Monday for five years, I can say with confidence that many brands sell out of their top sellers quickly. My number one tip? As soon as something catches your eye, add it to your…