[Closed] Opened box new phones on ebay.....

Did I just do something stupid? Need a new phone and decided to go sim only and buy a handset this time as so much cheaper. Saw a Samsung S20 FE for a good price, offered a bit less and it got accepted. "Result" I thought and paid. Then started thinking about it a bit more, quick Google throws up cautionary tales of stolen phones blocked by the networks or even people getting them on contract, flogging on and then failing to pay the monthly fee resulting in the phone being blocked, sometimes months later. Tell me this isn't common please! I assume most are people who have had a "free" upgrade and kept their old handset?