I've seen a few people mention that "the auction is offchain," but in fact all the auction mechanisms are in a smart contract that handles bidding, escrow, refunds, and settlement. Similar to Nouns, Zora Auction House, and Mirror.

Bids are authorized with an offchain signature in order to limit participation to Farcaster accounts. (At least for now). But the mechanism and logic are transparent and onchain.

Has been fun to revisit an old idea (ascending escrowed auction with refunds) with cheap gas and much better tech on the frontend.

https://github.com/farcasterxyz/collectible-casts/blob/main/src/Auction.sol

by the way, because auctions are onchain and events are indexed by FIDs, you can build pretty good tools for monitoring ongoing and interesting auctions yourself. lots of mini app opportunities here.