Let's start from the beginning.
As dhn says, you must associate this unit with a new email address, and should not use your old email with Home when attaching this unit. Home remembers your old serial number + your old email = your account and device.
Second, when hooked up to your PC, please review the size of the device under My Computer.
Next, understand that you can NOT load your old maps from your previous unit onto this one. Maps are keyed by serial number to specific units. Any attempt to restore your old maps to your 'new 910' will fail, and if you have overwritten maps that were already on this 'new 910' in the process, I certainly do hope you have backed this unit up first!
The explanation you have been given actually does make sense. The serial number is embedded in the board's flash memory. It is entirely possible that someone has transplanted a 710 board into a 910 when the 910 board failed. This would cause only confusion for us here, not a problem for you. The seller is correct that the boards are the same -- EXCEPT for the inertial sensor that guides you when you are out of satellite range (in tunnels, etc.). That piece would not be installed on a 710. Not a huge deal, but be aware of it.
So please answer those questions above. I am concerned that you could have overwritten the maps already on the unit without a backup of the unit -- and that would be a major problem.