How do I get to Daliushu Flea Market from Beijing China Exhibition Center by bus or subway?

Do you mean Beijing Exhibition Center or China International Exhibition Center? If your beginning is Beijing Exhibition Center, you should walk 460m to take metro Daxing line 4 from Zoo Station to Caishikou Station and take metro line 7 to Louzizhuang Station. Walk 300m to reach Daliushu Market.
If you set off from China International Exhibition Center, you should take bus no. 302 from Jinganzhuang Station to Tianshuiyuan Street Beikou Station and take bus no. 677 there and get off at Chaoyang District Daliushu Station. Walk 200m to the market.