Hold an item in your hand and use /chestshop create <amount of items> <buy price> <sell price> - use 0 if you want to disable the buy or sell price. To remove a shop, use /chestshop remove. You can also view shop information using /chestshop info.
Good memories, will always cherish them.
anyhow, I've already posted this in another thread but here we go again, enjoy this clip I made on OPBlocks v1: