People trying to access Genesis Market on Wednesday saw a screen saying, "This website has been seized" and "Operation Cookie Monster", along with a picture of a person in an FBI hoodie in front of a computer.
A cookie is a piece of computer data that makes it easier to reopen web pages.
Europol said the site offered "bots" for sale that had infected victims' devices through malware or other methods.
"Upon purchase of such a bot, criminals would get access to all the data harvested by it such as fingerprints, cookies, saved logins and autofill form data," it said.
The information was collected in real time so buyers would be notified of any change of passwords.
Prices for bots ranged from as little as $0.70 to several hundred dollars in the case of valuable bank account information, Europol said.
Unlike so-called "dark web" services, Genesis was available on the open web "although obscured from law enforcement behind an invitation-only veil", the agency said.
"Its accessibility and cheap prices greatly lowered the barrier of entry for buyers, making it a popular resource among hackers."
