You can think of the Ethereum blockchain as the gallery where your artwork is hanging. OpenSea is one window looking into the gallery. You can sell your items from any window to the first buyer who meets your terms, but once it's sold through one window, it's gone from the gallery and you can't sell it again through the other.
If you created NFTs using OpenSea, they'll only be visible on other platforms after the NFTs have been purchased or transferred. NFTs created on OpenSea aren't actually 'minted' onto the blockchain until they are purchased or sent to another wallet.
The process is called 'Lazy Minting' and you can read all about it on our blog post, Create NFTs for Free on OpenSea.