Chris Chapman used to own one of the most valuable commodities in the crypto world: a unique digital image of a spiky-haired ape dressed in a spacesuit, the New York Times reported. Chapman bought the nonfungible token (NFT) last year, as a widely hyped series of digital collectibles called the Bored Ape Yacht Club became a phenomenon. In December, he listed his Bored Ape for sale on OpenSea, the largest NFT marketplace, setting the price at about $1 million. Two months later, as he got ready to take his daughters to the zoo, OpenSea sent him a notification: The ape had been sold for roughly $300,000. A crypto scammer exploited a flaw in OpenSea’s system to buy the ape for significantly less than its worth, said Mr. Chapman, who runs a construction business in Texas. Last month, OpenSea offered him about $30,000 in compensation, he said, which he turned down in hopes of negotiating a larger payout. The company has made “a lot of stupid, dumb mistakes,” Mr. Chapman said. “They don’t really know what they’re doing.” Chapman is one of many crypto enthusiasts who have raised questions about OpenSea, an eBay-like site where people can browse millions of NFTs, buy the images and put their own up for sale. In the last 18 months, OpenSea has become the dominant NFT marketplace and one of the highest-profile crypto start-ups. The company has raised more than $400 million from investors, valuing it at a staggering $13.3 billion, and recruited executives from tech giants like Meta and Lyft. But as OpenSea has grown, it has struggled to prevent theft and fraud. The glitch that cost Mr. Chapman his ape has led to months of recriminations, forcing the start-up to make more than $6 million in payouts to NFT traders. Customers also complain that OpenSea is slow to block the sale of NFTs that were seized by hackers, who can turn a quick profit by flipping the stolen goods. And plagiarized art has proliferated on the site, outraging artists who once viewed NFTs as a financial lifeline. The company is facing at least four lawsuits from traders, and one of its former executives was indicted this month on charges related to insider trading involving NFTs.