My Sister Sold Magic Cards. Now The Card Shop Says They’re Counterfeit

So my sister decided to sell her mtg cards and the shop quoted her a total of 42,000 USD for the lot. Many were from the 90’s and were quite valuable. Now, the shop says they’ve determined they were counterfeit and refuse to pay or give the cards back. They have yet to provide substantial proof that they were fake
From my understanding, they have no legal grounds to hold the property. Obviously it would be illegal for my sister to sell potentially fake cards, but we want to have them tested by an independent party to the sale.
We do have reason to believe they may be real. The card shop just so happened to have listed the same cards for sale on their website after receiving my sister’s lot of cards. This could be a coincidence, but something rubs me the wrong way. Any advice would be appreciated!
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