United Factions features a server shop at which you can buy and sell certain blocks, items and equipment.

To access the shop, you need to type in the command: /shop

This will open an inventory menu as shown in the figure below:

(NOTE: Are you on Minecraft Pocket Edition and does the inventory menu look weird? Follow this tutorial to fix it: Fixing Minecraft Pocket Edition Inventory Menu)

The shop is subdivided into different sections. You can access one by left-clicking on it.
When you're in a section, you can buy using left mouse button, and you can sell using right mouse button.

The rest should speak for itself.

Extra commands

Sell all items of a specific kind in your inventory:
/sellall {material}

Sell all items you are holding in your hand
/sellall hand