If all segments are purchased or a specific segment isn’t available anymore you still get the chance to purchase it on the secondary market directly from the owner. You can do it either directly on millionpieces.io by selecting the segment on the world map or if the owner sells it on any external marketplace you can purchase it there. But beware of scammers who potentially created a copy of the NFT Certificate and want to sell it to you. Always verify (e.g. on Etherscan) whether it’s an original. That’s the beauty of blockchain technology.