A few things to consider:
The only way to get an idea of what performance to expect is to open it up and see what the tubes are.
The PVS-15 is an outdated goggle. There are much better articulating goggles available.
If you haven't used night vision before, or used/seen different tubes of varying quality, it will be hard for you to determine how good the tubes are. Even poor quality gen 3 will look amazing to most new users which is a double edged sword for them. It'll be awesome as a newbie but if you continue on with the hobby you may regret it later.
With all that said, depending on the trade details, condition & quality of the housing/optics/tubes, it could also be a good trade. At the minimum I'd want to know what the tubes are since they are the heart of the system.
ETA: you might be able to look into the rear eyepieces and read the tube info. It’s worth a shot.