Buying a used Garmin or TomTom GPS is a shot in the dark.

When I have a friend looking for a cheap GPS unit I take along a SD card with maps on it.
I test the unit in front of the person trying to sell it.
If it does not work I walk away from the deal.
There are many good deals to be found at flea markets and private sales.
But only if you test the GPS ahead of time.
Other than that it is a waste of money and time only to find out the GPS has been screwed with to a point it does not work correctly.
I never buy any used GPS over the internet for someone.
They make good paper weights at best.

"bam-bam" has also asked for help over at "GPSURL"

Good luck on getting it to work as we do not know what was done to the Nuvi 50 before you got your hands on it.
These forums of full of screwed up Garmin and TomTom GPS units that some one thought they got a great deal or did not know the seller tried to change the GPS unit to some other model.
Then they just sold it off to an unsuspecting user.