Buying more weapons costs Gold. So check your Stats on the right to see how much Gold you have.
For example, for every additional sword, you will have to pay 75 units of Gold.
Helmets and Shield help your Defense. Swords help your Attack.

By choosing to sell some of your weapons, you will get gold back: but not as much as you paid for it!.
For every sword you sell, you will only get 50 units of Gold back.