In this case you are tasked with designing and building a simple off-chain system and the on-chain smart-contract to enable cheap auctions. The goal is to enable trades between an ERC721 and a ERC20 with a single on-chain transaction. The auctions should work as follows: - Owner of the NFT approves all NFT’s to the Marketplace - Owner of the NFT signs to create an off-chain auction listing with a minimum price - Bidder approves ERC20 tokens to Marketplace - Bidder signs a bid for the auction - If owner approves the bid, signs it back and retrieve to bidder - Anyone with both signatures can settle the transaction, the owner takes the ERC20 whilst the bidder takes the NFT.