What steps can be taken to resolve property sold without permission, following a death?

My father died after complications during heart surgery. He left behind a large inventory of rare books that were sold off through an antiques center(where he had a business arrangement) without permission from his estate. There was an offer made to purchase the inventory, but no contract signed or verbal agreement. The only reply given was that there was a check for $2,300 should we be interested in settling it for that amount. The check was never picked up. An agreement was never made. When a family member went to discuss this with the business owner, she was told that the books had all been sold, & that there was no inventory list available, nor would they tell her the amount that the books were sold for, or who they were sold to. The books were valued at well over $10,000. Due to my father's death, we have yet to find an inventory list. Is it legal that the books were sold without permission from the executor of the estate, which I am. Please advise. Thank you.