By Bethan Shufflebotham I’m calling it. This Marks and Spencer bag is going to sell out \- and fast. And if you want my opinion? I’d buy it now, and be smug about it later when every social media influencer and their dog are trying to hunt it down on Vinted. I’ve been writing about fashion for seven years, and during that time have developed a knack for knowing exactly which pieces from a retailer’s new-in collection will go viral, sell out, or end up better than half price in the sale months later. But I’m particularly talented at spotting Marks and Spencer gems. Last year, it was that stripe…