Hey there
If you are thinking of selling your device you have two options.....
1. Search Mobile Phone Recycling Comparison on Google and choose one of the many recyclers on there. Look for those with good reviews and remember to be honest and accurate in your appraisal of your device to ensure you actually get the quoted price. Expect around £500 or so, maybe slightly higher... That said, as you've an international version when you put your IMEI in on the chosen site you may get a lower price.
2. Sell privately either to friends or for cash. For flagship devices I'm not a great fan of ebay. There are a lot of scammers out there who will take the device then claim you sent an empty box etc.
You may get slightly more but the risk is higher.
S24 Ultra