Pawn Shop Wants Me To Pay Them Back Or They’re Taking Legal Action

I sold some graded Pokemon cards to a pawn shop in Colorado in late December of 2024. I was very new to trading cards in general and didn’t know much about graded cards at all. The pawn shop called me today which is a little over a month from the sale date saying that they’re not legit and is saying that I need to pay him back or he’s taking legal action. I had no clue that they were fake and didn’t even know that fake graded cards were even out there! I did till him that I was new to the trading card scene and that I didn’t know too much about the cards besides how much they go for on the market since I looked them up. What should I do? Thank you!