DENVER — In a world where Amazon can track your next book purchase and you must register to buy allergy medicine, James Holmes spent months stockpiling thousands of bullets and head-to-toe ballistic gear without raising any red flags with authorities.

The suspect in the mass theater shooting availed himself of an unregulated online marketplace that allows consumers to acquire some of the tools of modern warfare as if they were pieces of a new wardrobe. The Internet is awash in sites ranging from BulkAmmo.com, which this weekend listed a sale on a thousand rifle rounds for $335, to eBay, where bidding on one armored special forces helmet has risen to $799.

"We're different than other cultures," said Dudley Brown, executive director of Rocky Mountain Gun Owners, which advocates for firearms owners' rights. "We do allow Americans to possess the accoutrements that our military generally has."