Weapon store

The list of all the weapons that can be bought is on the left.
If you already own the weapon, it will be highlighted.
There are 2 types of weapons: primary weapons have a red background and secondary weapons have a blue background.
Clicking on a weapon will display its characteristics in the information panel.
In this panel you can buy the weapon (or sell it if you already have it) and its ammunitions.
Secondary weapons don't have ammunitions, you just have to buy multiple items of that type.

The player currently owns a rifle and some dynamite. They are both equipped. The info panel is displaying information about one of the available guns.

The info panel displays information about the rifle owned by the player. You can then buy more bullets or sell the rifle to recover some money.

As already said, if you own a weapon this is highlighted in the list on the left. If you want to bing a weapon with you, you have to drag it from the list on the left and drop it in one of the slots on the right.
As you can imagine, the red slots will keep primary weapons, the blue ones secondary weapons.
The 2 red slots correspond to the keys 2 and 3, those blue correspond to the keys 4 and 5.
You cannot unequip or sell your bowie knife: you will always carry it with you in your missions and it can be accessed in game by pressing key 1.