how to interact with an already deployed smart contract

Question

I was reading this contract https://docs.soliditylang.org/en/develop/solidity-by-example.html#safe-remote-purchase that I found in the solidity documentation and I assume this contract is unique and deployed by the auction house. All users can make an offer. From an implementation point of view, how does a user in web3.js call the methods of this contract if he didn't deploy it?

Answer 1

By reading at the contract, there are essentially two roles:

> Seller: this is assigned inside the constructor, meaning that only the person who deployed this contract becomes a seller:

constructor() payable {
    seller = payable(msg.sender); // Contract creator becomes seller
    value = msg.value / 2;
    require((2 * value) == msg.value, "Value has to be even.");
}

> Buyer: it can be anyone who performs a purchase, as described in the function below:

function confirmPurchase()
    public
    inState(State.Created)
    condition(msg.value == (2 * value))
    payable
{
    emit PurchaseConfirmed();
    buyer = payable(msg.sender); // Anyone -successfully- calling this function becomes buyer
    state = State.Locked;
}

Once the roles are assigned, there are other functions within the contract that can be called only by the seller or buyer depending on the function modifiers (onlyBuyer, onlySeller).