If you are not willing to sell the plate for the price, you will need to contact the buyer and explain what you have written here. But be aware, you may have one unhappy buyer on your hands

Other members here cannot help you, it is up to you as the seller to understand how auctions work before you start listing things 

The plate did not 'somehow sell' it sold because you listed it  it , it got 20 bids and sold to the highest bidder