Following a vulnerability on the site, OpenSea has repaid about $1.8 million to its users after allowing people to buy and sell NFTs for little. A number of blue-chip NFTs, including Bored Ape Yacht Club, Mutant Ape Yacht Club, Cool Cats, and Cyberkongz were resold for well below market value this week without the seller's consent.

The miscalculation was initially reported as a bug, but OpenSea has since acknowledged that it was caused by a mechanism that had been exploited by skilled traders.

“When a user transfers an NFT out of a wallet (“Wallet 0xABC123”) while “Wallet 0xDEF456” is not online, we send messages to Wallet 0xDEF456 saying it owes this NFT to Wallet 0xABC123,” OpenSea explained. “If the wallet is misconfigured, it ignores this message. The user will still have a record of their NFT in Wallet 0xDEF456 but no other evidence that it exists.

In other words, OpenSea initiated transactions from Wallet 0xABC123 to Wallet 0xDEF456 for each of its NFTs, but Wallet 0xDEF456 was never online to process the transactions.

“Wallet 0xDEF456 has no record that it received any NFTs from Wallet 0xABC123, so the user is still believed to have all of these assets. The user can use this to their advantage, by buying out Wallet 0xDEF456’s assets and putting them back on the market for sale.

The NFT marketplace fixed the issue shortly after users shared pictures of their profiting from the exploit via Twitter. Trading of Cool Cats was stopped “for some time” while traders like CryptoKongz took to social media to share their profits.“I feel like a complete dumbass now for paying over 10 eth,” CryptoKongz said on Discord. “But it’s a lesson learned and now I know for sure the smart way of selling stuff, wait until they are all out!"