I intend to buy ladies bags and western gowns where should I guangzgou or Yiwu

Oh, both these two places are ok. YW has lower prices. Guangzhou has better quality. But this doesn't mean the quality is poor in YW, just not as good as in Guangzhou.
In Yiwu, you can go to YW International Trade City.
For bags, you can find lots of stores located at the 1st Floor of Second Area. The trade city is located at east side of North Houzhou Road. For clothes, you can go to Huangyuan Clothes Wholesale Market. Most clothes there have good quality and cheap prices.
In Guangzhou, you can go to Baima Clothes Wholesale Market. It's located at No.16, Zhannan Road, Yuexiu District.
In addition, go to Jinyi Leather Products to buy bags. The address is No.1453, North Jiefang Road, Baiyun District.