These markets are the main places to purchase local products, and products from the region (Vietnam/Thailand) to take home as souvenirs.
Both markets have numerous stalls which sell clothes, kramas (Cambodian scarves), woodcarvings, silver jewellery and other textiles.
The Russian market is smaller and the stalls are cramped and dark. A visit to the markets is like diving into a dingy, dark hole, and on humid days, it is rather suffocating.
The Central market is my preferred choice, as it is brighter, airier, and has wider walking areas. It also has a wider range of products to purchase and has more books. Some people do say that it is also more pricey.
In any case, bargaining is the order of the day. But seeing the state of some of the Cambodians, especially landmine victims who hawk books and postcards for a living, I avoid being mercenary when making a deal.