LeBonKoin

LeBonKoin is a decentralized marketplace inspired by LeBonCoin, enabling users to buy and sell physical goods between private individuals using USDC.

Users authenticate via World ID to guarantee their uniqueness, make multichain payments using Circle's CCTP V2, and validate their transactions transparently via Clear Signing on Ledger, according to the ERC-7730 standard.

An escrow contract guarantees payment security until confirmation of receipt by the buyer.

Frontend: React + Next.js, with integration of the World ID widget.

Backend: Node.js (Express), used to verify World ID proofs via Semaphore.

Smart Contracts: Solidity on Ethereum L2 (Base/Arbitrum), including :

USDC escrow contract,

ERC-7730 compatible functions for Clear Signing (structured JSON),

direct integration with Circle CCTP V2 for fast multichain transfer.

USDC & Circle :

Use of Circle CCTP SDKs to enable the user to pay from any supported chain (e.g. Avalanche → Arbitrum).

Funds are bridged via CCTP before being deposited in the escrow contract.

Ledger:

Implementation of the ERC-7730 standard on key functions (buyItem(), releaseFunds()) to enable a readable and secure signature on Ledger.

World ID:

The user authenticates via World ID.

Proof is verified in the backend before any sensitive interaction (purchase or ad publication).

Off-chain storage:

Ad metadata is stored on IPFS via NFT.storage (images, descriptions, prices).

Hacky but effective: to get around the lack of native multi-chain rebalancing tools between users, we've encapsulated the CCTP bridge in a user interface that detects the buyer's chain, sends the USDC to the escrow via Circle, then triggers the purchase on the target chain.