A dark web marketplace is an illicit online shop, usually reached through anonymity networks, where criminals buy and sell stolen data, malware, credentials, and other illegal goods. These markets act like underground exchanges: sellers list items, buyers pay, and trust is built through ratings, escrow, and repeat trade.
In cyber security, these marketplaces matter because they turn a single breach into reusable criminal inventory. Exposed Social Security numbers, passwords, session cookies, and business records can be combined and resold for phishing, account takeover, business email compromise, and fraud. Defenders use this intelligence to monitor for leaked credentials, investigate exposure, and strengthen verification for sensitive requests. Because listings can be copied and repackaged, removing the original leak does not always remove the risk; once data enters a dark web marketplace, it can keep fueling attacks for months or years.