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. Markets can also be altered by the Workers of the Week.
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!