OPN Marketplace & DeFi Suite

What we built

OPN Marketplace & DeFi Suite is a full on-chain economy on OPN Chain, combining an NFT marketplace, an AMM token swap, a staking system and a referral airdrop into one connected product. Everything is permissionless: anyone can mint, trade, provide liquidity, stake, or claim — no gatekeepers, no off-chain settlement.

Why

Most "marketplace" demos are isolated features with no economic loop. We wanted real open-finance infrastructure where each module reinforces the others, the project token has genuine utility, and value flows transparently on-chain — a self-sustaining mini-economy rather than a disconnected demo.

Contracts

Six contracts form a single value loop:

ProjectToken (OMT)  - ERC-20 utility token, hard cap + controlled minting
NFTCollection       - open ERC-721 minting + EIP-2981 creator royalties
Marketplace         - list / buy / cancel / offer trading + fee split
SimpleSwap          - x*y=k AMM for OPN <-> OMT, 0.3% LP fee
Staking             - stake OMT to earn marketplace fees in OPN, no admin key
AirdropReferral     - claim OMT + reward on-chain referrers

How the value loop works

NFT sale -> 2.5% platform fee -> Staking contract -> OMT stakers earn OPN

This gives the token real demand and makes the whole system sustainable. Every sale also pays the creator their EIP-2981 royalty before the seller is settled.

Technical execution

Solidity 0.8.24 + OpenZeppelin 5, compiled for maximum OPN compatibility:

solidity: { version: "0.8.24", settings: { evmVersion: "paris" } }

• Reentrancy guards, owner/approval checks, and capped royalties/fees throughout.
• 22 end-to-end tests passed live on OPN Testnet across two wallets (token transfer, NFT mint, list/buy with correct royalty + fee split, offer/accept trade, AMM swaps both ways, staking reward distribution, referral rewards).
• All 6 contracts are source-verified on the OPN Blockscout explorer.

Real-world viability

Built-in revenue model (2.5% marketplace fee + 0.3% swap fee), creator royalties, and a growth loop (airdrop + referral). The frontend is a Next.js 14 dApp (wagmi + viem) covering create, market, swap, stake, airdrop and a portfolio view — ready to scale from proof-of-concept toward live infrastructure.