There are four types of marketplace in Avalon: regular shops, commodity trading stores, village markets and Old Curiosity Shops. The Old Curiosity Shops are a special case and detailed in HELP SHOPPING. The rest are stocked by Avalon players, city ministries or villages from their innate production and trading.
Regular shops tend to be stocked by players and are the cornerstone of invaluable equipment; see HELP SHOPS for detailed info. Commodity trading stores are where the cities and villages centrally buy and sell the raw commodities from which equipment/items are made. There is one per city or village. Village markets are short-term affairs, cropping up either at certain times of the day or certain dates in the Avalon year (see HELP CALENDAR). The most useful aspect of the village markets is their demand for certain materials and commodities you may possess and wish to turn into real gold coins.
Village markets are usually set up close to the centre of the town, or on the main square, and there are two distinct times for market trading: noon and evening. Foodstuffs and perishables are sold before the aftenroon, while valuables and long-lasting commodities are sold in the evening. From your perspective, as a potential trader, the two most profitable market vendors are the fish and meat traders before the afternoon, and the gems/precious metals traders in the evening. Many villages have both varieties sharing marketplace space through the day.
To trade the core foodstuffs use SELL <amount>/ALL FISH TO MERCHANTS or SELL <amount>/ALL MEAT TO MERCHANTS in the noontime markets, and same syntax with GEMS or whatever precious stones for the evening markets. It is crucial to include TO MERCHANTS to indicate you wish to direct your trade to marketplace merchants since often markets share location with commodity trading centres. The number of gold coins you receive will depend on recent trading activity, over the session and over the recent months. Arriving early in a session is often the best way to guarantee a higher price.
See HELP SHOPPING, HELP TRADING and HELP SHOPS for related helpfiles.