What do all auction companies in Zhongshan have?

Up to now, there are 20 auction enterprises in Zhongshan. They are: Zhongshan Kangxin Auction Co., Ltd., Zhongshan Zhongyu Auction Co., Ltd., Zhongshan Bao Zhong Auction Co., Ltd., Zhongshan Xinkang Auction Co., Ltd., Zhongshan Jinan Auction Co., Ltd., Zhongshan Zhengming Auction Co., Ltd., Zhongshan Xinggang Auction Co., Ltd., Zhongshan Yezheng Auction Co., Ltd., Zhongshan Jinying Auction Co., Ltd., Zhongshan Bing Zheng Auction Co., Ltd., Zhongshan Hongtai Auction Co., Ltd., Zhongshan Sotheby's Auction Co., Ltd., Zhongshan