Less than 50 rounds?
This is not a car where mileage/kilometers count... it's a Glock!
I wouldn't pay more than half the price of a new Glock.
Usually those are sold here for around €300 (a new one costs €665).
I believe that in the US a new Glock costs about $550... so maybe about $250 for that 1st. gen.
There are lots of 1st gen. Glocks out there so it's not a collectors item... it's just an old gun.