Thousands of iPhone X models are listed, many that already have tons of bids. An auction ending in an hour for a 256GB iPhone X has a current bid of $1,675, for example, a premium of $500.
It's not really a premium of $500 unless you live in a no sales tax state. Otherwise it's more like a $450 premium over what you'd pay in a store. And the buyer only gets to keep about $175 of that, the rest going to shipping, eBay/Paypal. Hardly seems worth the effort unless you had a entire cases to sell. Looking at ended auctions seems 1600-1700 is the going price with a smattering above that.