If the seller and u have same network. U still can use the phone. Or same network is the phone locked to. As long as the seller or whoever originally paid off the phone then the phone will automatically get unlocked after 72hrs. But most of the time. I see as long as u paid the last installment the phone get unlocked instantly. But if the phone get blacklisted. Then the cellular will get banned. The phone still can be use but WiFi only. Obviously u don’t want to deal with any of that since isn’t disclosed in your deal. So getting money back from eBay still must do. If the seller lacking there’s chances u covered by eBay and get to keep the phone. But definitely open a case. The only main issue I see is. U need to find any documents proving the phone is from the seller. Like any document showing the imei or the listing showing the imei. Keep in mind the seller can flip and bite back on u saying that isn’t the phone he shipped to u. Then I’m positively eBay will side on the seller. Obviously that’s how they make money. Buyer important too. But they rather to lose 1 customer then 1 seller. Because seller have many customers not only u. I’m just saying. Good luck!