The server's economy is based around diamonds. 1 Diamond is equivalent to $100 and when a purchase is made at a shop it will automatically do the conversion for you.
You need to make sure that you have diamonds in your inventory when shopping at players shops. It is encouraged that shop owners have an enderchest readily available for players to use to access potential stored diamonds.
Use the command '/shop' or '/warp shop' to teleport to the shop!
A few important economy commands are:
/money or /bal
Shows you your balance.