The market allows you to sell goods collected on the farm and buy necessary goods. The market building becomes available at level 12. Additional boxes for selling goods at the market can be purchased for Westbucks.
The merchant works exactly like a drugstore - you pay for his services, and he looks for any items you need (except horseshoes, expansion and improvement items). After completing the order he will have 1 hour rest - it's not the easiest thing to manage the boat! You can hire a trader for 1, 5 or 10 days. The merchant appears at level 14.