A marketplace for buying and selling non-fungible tokens (NFTs) has shut down the majority of transactions due to the proliferation of counterfeit material, its founder announced late Friday. Cent CEO Cameron Hejazi said that his company had halted sales of nearly all digital assets starting Feb. 6 because users kept on “minting and minting and minting counterfeit digital assets.” Hejazi identified three problems that had plagued Cent: unauthorized copies of NFTs, tokens based on art and assets the NFT creator did not own, and tokens meant to act as a financial security. He added, “There’s a spectrum of activity that is happening that basically shouldn’t be happening—like, legally… It kept happening. We would ban offending accounts but it was like we’re playing a game of whack-a-mole.” He called the plagiarism and the minting of illegal NFTs a “fundamental problem” for NFT marketplaces. One type of sale is still allowed on Cent, though: NFTs based on tweets. The company made its name by selling a token of Jack Dorsey’s first tweet for $2.9 million in October of last year.