If I list my work on Manifold’s Gallery and someone makes an offer on OpenSea, I can accept the offer on OpenSea and it will be sold there, correct?
No - The Manifold Gallery is a custodial auction, similar to FND when it’s listed for sale it’s transferred to our Gallery to facilitate the sale. You won’t be able to list a token for sale on Gallery and in Opensea at the same time.
What happens when someone makes an offer on OpenSea for something I list on Manifold’s Gallery?
Gallery auction will need to be ended and the NFT withdrawn before you can accept the offer on OS.