2nd hand garmin card

[83XT4]

HI all
Have seen a garmin chip on ebay and was curious as if this would be value as i understand that they work via a 'unlock key' for a specific gps, plus a 2nd.
So if i buy said card it will not work on my 2nd gps as it has already been used on a different gps.
Garmin seem a bit harsh as if i transit one area, eg pacific islands, i cannot even give my chip away as it is keyed to my specific gps.

Is this correct and there fore no point buying chips on ebay.

Thanks guys