I haven’t had to do this for a while, and only as a buyer, but you can start a grievance and get the persons phone number. (Assuming it’s real) start harassing them, asking them where your money is, also you have his address, he has stolen from you. Call the cops.
I phoned a seller once, and got his dad (turned out he was 13) his dad gave him a dressing down over the phone, made him apologise to me and then I got the item 2 days later by special delivery.
I know a guy who bought an item and it was never sent. He ran his own company and had a couple of reps in the area the next week. He told the guy he was sending a couple of boys round to see what the delay was with postage (pretty much insinuating he was a gangster). When the reps showed up, knocked the door and explained who they were he got the item very quickly.
Also report to ebay, might get the user banned. I do hope you have some evidence of postage? There’s a good chance that if they’re a dodgy user they’ll pay, then instantly request the money back from ebay claiming you never sent the item. If you don’t have proof of postage ebay will just automatically side with the buyer.
Worst case (as a seller) is when someone hacks someone else’s account, and buys something. Ebay freeze the payment and then return it. Often often you have sent the item. You lose out here too. Through no fault of your own.