You never want something until you can't have it. So, each week in The Sell-Out, we're getting the scoop from your favorite retailers on what's selling like crazy. Watch this space to find out what everyone's buying, sign up for wait lists, and keep tabs on restocks.If you've ever shopped at Forever 21, you know its merchandise selection moves quick; it's rare that you'll come across a shelf filled with the same exact clothes you saw there a week ago. With so much movement (and thousands of pieces available), it's not easy to tell which particular items are the most popular (and most difficult to get your hands on).Turns out, there isn't just one or two goodies that are selling the quickest — but an entire section. Forever 21's Last Chance To Buy page (adding this to our bookmarks tab immediately) is where the store's most devoted and savvy e-comm shoppers head first (and we're about to start doing the same). Not only is the selection filled with basics and accessories that everyone clearly loves, but it also lets you know which items from outside brands Forever 21 carries are low in stock (like this amazing throw-and-go frock from The Fifth Label). Sure, it's a bit of a higher price point than you're used to seeing at the fast-fashion retailer, but its versatile print and flattering silhouette make it the ultimate dress-up-or-down piece for your closet (which means you'll definitely get your money's worth).Right now, the dress is still available in sizes small and medium — but we're betting it won't be in stock for long. So if you're currently spring dreaming, this simple piece will help you get there even faster. And if it's already sold out by the time you're reading this, we've rounded up some similar picks to shop.