Markets in-game fluctuate depending on supply and demand. This means the markets go up or down based on trading between players and at shops. The market rates determine how much a store is willing to pay for your items as well as how much they will charge you for products. Most markets update every 6 hours in-game.
Higher turnover will decrease the market rate due to higher supply. Vice versa, lower turnover will increase the market rate since supply is low.
Simply monitor the market using /market and complete missions and activities with a high market rate. If the fishing market is high, sell your fish for extra profit! If the housing market is low, treat yourself to a new house!