A good test for a used GPS is to put batteries in it, let it locate itself, enter a few waypoints, then shut it off and take the batteries out. After about 5 minutes put the batteries back in and see if it remembers its location and the waypoints.

The most common reason GPS receivers go bad is the internal battery fails. The internal battery serves the same purpose as the battery on the motherboard of your computer. It looks like a watch battery much like the ones in computers. It keeps the chip memory alive.
When the internal battery fails in a GPS you lose the memory features and the GPS will need to recalculate its location and every time you turn it on and will lose its waypoint registry.

They are replaceable but it is not usually a do it yourself job because the battery is usually soldered into the battery clip on the board.
May set off to explore without any sense of direction or how to return.