I have long been skeptical about sites like StockX, but used it as a last resort in December. Worked out well in my case, at least.
I can't guarantee the shoes I got are legit, but they appear to be. Came with a model # traceable to the manufacturer, and my household scale showed they were approximately the same weight as the manufacturer's listed specs. Felt bouncy, and are holding up as super shoes typically do with 100 miles to date.
I'd have preferred to buy from an authorized reseller or store, avoiding the question altogether. But I needed a new pair of racing shoes for a January 2024 marathon, and all stores were sold out of common sizes of just about every brand in December. I presume that was due to the holiday rush.