Contact the seller first. This is the fastest path — ask them directly for the UEFI password or to have it removed before you accept the item.
Many Surfaces sold at auctions come from businesses or schools that set a UEFI password before disposing of the device.
Contact Microsoft Support. They may be able to tell you whether the device is still managed or under warranty, but Microsoft generally cannot remove or recover a forgotten UEFI password for security reasons.