When issues come up - there are two directly opposite ways to solve problems. You can opt for either drama (which will probably be damaging) or for honest communication (which will probably result in harmony and cooperation). Setting up for estate sales is a lot of work, you won't always have other people who understand how estate sales work or what exactly you do for your clients. Being an estate sale business owner requires a lot of diplomacy and communication, here is how to work and solve problems to your advantage as an estate seller.
I had an estate sale I was conducting for a moving and storage company. Their client had to sell everything to pay their overdue storage bill. They referred the job to me and I set up a contract that authorized payment to the storage unit first and then to the client after the sales. The storage company owner removed items she thought were valuable to get them sold. That was not in the contract. I was angry that she was trying to cut me out of a sale, I had set up a contract and her bill was going to be paid. I wanted other sales from her in the future. Even though I was annoyed at her actions (she did not have a contract with the client to sell the items, I did), I called her and explained that she needed to bring the items back and why I had to handle all sales. She had no idea what the items were worth or who could appraise them or buy them. I was the person who had that job.
Another seller has had a similar issue with a realtor taking items from an estate. Some people say to call the police, or to report them to the Board of Realtors. Here is a facebook live post with my take on these issues.