Core/PacketIO: Updated CMSG_BUY_ITEM structure and opcode.
Updated SMSG_BUY_ITEM structure and opcode.
Updated SMSG_BUY_FAILED structure and opcode.
Updated SMSG_INVENTORY_CHANGE_FAILURE structure and opcode.
Updated CMSG_SELL_ITEM structure and opcode.
Updated SMSG_SELL_ITEM structure and opcode.
Updated CMSG_BUYBACK_ITEM structure and opcode.
Updated CMSG_REPAIR_ITEM structure and opcode.

Thanks to @zalito12 and @projectcore for various parts of this commit =)