Unfortunately buying from Ebay can present issues sometimes.
May I ask why did you need to take it to a shop ? Is there something wrong with the phone ?
Has it been imei blacklisted and as such blocked ? If so there's nothing you can do about this.
Please don't try to resell it on Ebay as this is just potentially passing this onto some other unsuspecting person which isn't fair !
Try looking at your Ebay reoksution centre and or raise a dispute with PayPal if you paid that way.
Other than that you may have to chalk this one to a bad Experience.
Current Phone > Samsung Z Fold³ 5G. > Software : Android 12 ~ Samsung One Ui 4.