You should have grayed dialog line if u already played with specified merchant. This dsn't mean that you won a game, but if u play until you win every time, that should give you info u need. It's working even after load / save.
i read about this app someone posted on ps3trophies.org on google play called gwentcheck its what im using. has pics, check lists, what they do and where to get them everything.
Edit: I just used it and i was missing 5 cards 4 which were all monster cards you have to buy from an inn that i missed then i got the trophy. it still says im missing a ballista card though, oh well. i thought for sure i missed that first card alot of people say they missed. was gonna start over, but thought id wait to see if they would patch it good thing i didnt.
...but just a fyi there are cards with a certain one way pass you must do that unlocks for a short window in a certian place in the story. ...