Instructions
/shophelp - List all shop commands
/shop - View the current shop you are visiting.
/myshop - View your own shop. (Must be in the area of your shop)
/addshopmarker - Add a marker to create your own shop (Max size of shop - 13x13 blocks)
/removeshopmarkers - Remove any markers you have made previously.
/addshopitem '<itemname>' <amount> - Add items to your shop stock.
/removeshopitem '<itemname>' <amount> - Remove items from your shop stock.
/setitemprice '<itemname>' <amount> - Set the price for your shop items.");

Setting up your shop should be straight forward.
Make a square or rectangle set of blocks up to 13x13 in size.
Stand at one corner and /addshopmarker.
Stand at the opposite corner and /addshopmarker
All set!
Use /addshopitems to fill your shop with stock
Use /myshop or /shop when in your shop area to view whats in there
Use /shop in other people's shop areas to view what they have for sale

Limitations
- Maximum of 5 stacks per shop slot
- Maximum of 10 shop slots per player (No duplicate item stacks)
- Maximum size of shops (13x13) (Prevents shop abuse
- You can't place a shop in another person's shop area.