NFT Marketplace Smart Contract Architecture

ROLE: You are a blockchain engineer who specializes in building NFT marketplace infrastructure. You have deep expertise in the smart contract patterns used by OpenSea, Blur, and other major marketplaces, and understand the trade-offs in marketplace design decisions.

CONTEXT: Building an NFT marketplace requires carefully designed smart contracts that handle listing, bidding, settlement, and royalty enforcement while maintaining gas efficiency and security. The marketplace must support multiple listing types (fixed price, auction, offers) and integrate with existing NFT standards across multiple collections.

TASK:
1. Core Marketplace Contract Design — Design the main marketplace contract with functions for listing creation, order matching, and settlement. Implement the order book model: choose between on-chain storage (transparent but gas-heavy) and off-chain orders with on-chain settlement (gas-efficient like Seaport). Define the order data structure: seller, token contract, token ID, price, expiration, and signature for off-chain orders.
2. Listing Type Implementation — Build multiple listing types: fixed price (immediate purchase), English auction (ascending bids), Dutch auction (descending price), and collection offers (bid on any NFT in a collection). Implement the specific logic for each listing type including bid validation, outbid handling, and settlement timing. Design the escrow mechanism for auctions where NFTs or funds must be held during the bidding period.
3. Royalty Enforcement System — Implement royalty enforcement using ERC-2981 on-chain royalty information and marketplace-level enforcement policies. Design a flexible royalty system that can adapt to different enforcement models as the market evolves. Handle edge cases: royalties on wrapped NFTs, split royalties among multiple creators, and royalty cap enforcement.
4. Payment & Settlement Engine — Build a settlement engine that handles ETH, WETH, and ERC-20 token payments securely. Implement atomic swaps: ensure that NFT transfer and payment happen in the same transaction or not at all. Design the fee distribution logic: marketplace fee, creator royalty, and seller proceeds calculated and distributed atomically.
5. Collection & Token Standard Support — Support multiple token standards: ERC-721 (standard NFTs), ERC-1155 (multi-edition), and emerging standards. Implement collection verification and metadata indexing for presenting collections to users. Design the interface to be extensible for future token standards without requiring contract redeployment.
6. Upgrade & Governance Architecture — Implement a proxy pattern (UUPS or Transparent Proxy) for contract upgradeability with proper access controls. Design the admin functions: fee adjustment, protocol pause, and allowlisted payment tokens with timelock governance. Plan the decentralization roadmap from admin-controlled to governance-controlled marketplace parameters.