SupplyChain.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;
...

    // 12th step in supply chain process
    function sellItemByRetailer(uint256 _productCode, uint256 _price)
        public
        onlyRetailer // check _msgSender() belongs to RetailerRole
        receivedByRetailer(_productCode)
        verifyCaller(items[_productCode].ownerID) // check _msgSender() is ownerID
    {
        items[_productCode].itemState = State.ForSaleByRetailer;
        items[_productCode].productPrice = _price;
        emit ForSaleByRetailer(_productCode);
    }
    
    // 13th step in supply chain process
    function purchaseItemByConsumer(uint256 _productCode)
        public
        payable
        onlyConsumer // check _msgSender() belongs to ConsumerRole
        forSaleByRetailer(_productCode)
        paidEnough(items[_productCode].productPrice)
        checkValue(_productCode, payable(_msgSender()))
    {
        items[_productCode].consumerID = _msgSender();
        address payable ownerAddressPayable = _make_payable(
            items[_productCode].retailerID
        );
        ownerAddressPayable.transfer(items[_productCode].productPrice);
        items[_productCode].ownerID = _msgSender();
        items[_productCode].consumerID = _msgSender();
        items[_productCode].itemState = State.PurchasedByConsumer;
        itemsHistory[_productCode].RTC = block.number;
        emit PurchasedByConsumer(_productCode);
    }
  ...