KHLONG LUEK-POIPET, THAILAND — THIS one-time minefield is now a thriving market.Less than four months ago, the narrow iron bridge linking the twin markets of Khlong Luek in Thailand and Poipet in Cambodia was deserted and barricaded with barbed wire. For almost two decades, the xenophobic rule of the radical Khmer Rouge and a civil war kept the border officially closed. Today, the frontier bustles as a hubbub of bare-knuckled commerce. Daily, thousands of traders, their rubber boots churning the monsoon mud, converge from as far as Bangkok, a four-hour bus ride away. Where there were once a few small open-air stalls, covered market pavilions multiply, crowded with shoppers. Just a few months ago, one could hear the thunder of artillery fire from Khlong Luek. Now, the empty field has become a parking lot jammed with pickup trucks, horse carts, motorcycles, cars, and auto-rickshaws. Four-wheel carts, powered through the mud by human muscle, shuttle shoppers between the twin markets. In the shops, porcelain elephants and other handicrafts from Vietnam are for sale next to liquors from France. Tools and military surplus from the Soviet Union are displayed with cosmetics and televisions from Thailand. One enterprising Thai has set up an international mobile telephone service for Cambodians to call relatives in other countries. On a recent steamy afternoon, a queue of people waited to place calls to the United States, Australia, and Singapore. The newly opened border has drawn thousands of Cambodian refugees in nearby camps. "Everyone is concerned with making a living, and money, and buying a sewing machine or something to take back home with them," says a United Nations official. However, on the Thai-Cambodian frontier, trade is an increasingly raw and brutal business. For years, the three guerrilla factions and Phnom Penh government soldiers have managed their own black markets, relying on the complicity of the Thai Army to let merchants across the border. Officially, the Thai Army has kept the border sealed, while it gave the Cambodian guerrillas sanctuary and channeled weapons to them from China. Today, robberies are on the rise with trade, observers say. On the Cambodian side, soldiers patrol the market. In Poipet, amputee beggars threaten and extort money from merchants, Western aid workers say. Just outside the town, some Cambodians have established a tollgate through which shoppers and traders must pass and pay, Cambodians say.