NFTs can be marked as fake, spam or inappropriate when the NFT contract has been flagged.
NFTs that are flagged can not be sold on the market, but can still be transferred.

The usual reasons why an NFT is flagged are:
NFTs that violate copyright rules
NFTs that are used to spam
NFTs that are used to push advertisements
NFTs that are fake and are used for to scam people

NFTs coming from a general minter contract such as the public minter of Polygon can also get flagged if that contract leads to many scam and/or fake nfts or if no contract owner can be differentiated.
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