In the market you can buy tackle, bait, food, home and other useful things.
On tab Tackle you can buy line, hook and rod. Each have max weight, some have and min weight. If your line max weight is too low fish will break it. For line there is no min weight. If your hook max weight is too low fish will break it. If your hook min weight is too high fish will cant be catched. The higher max weight of rod will facilitate pull the fish. But each rod have strength and when it go to 0 % rod will disappear whit used line and hook. TIP for this case: Have buyed cheap line and hook and often check in backpack on tab tackle the condition of your rod and when become little set your cheap line and hook.
On tab bait you can buy bait which what to catch fish. Without bait you can't cast. Whit different bait on different locations you will catch different fish. Click on the link for more description about Bait: http://gofishing.wikia.com/wiki/Baits
Here is the useful things. With it you can raise your fish limit in backpack, your max energy