So a potential buyer shows me his GWCL and it turns out to be fake, am I still responsible?You fixed that for yourself.
In a civil suit, you don't have many of the guaranteed protections from government, which you would have in a criminal court. Bad stuff that we couldn't readily foresee sometimes happens. Best not to kid yourself about such things.
If you are that worried then only a 4473 and background check at an FFL dealer would completely cover you...and even that's not 100%.