The lawsuit was announced Tuesday by attorneys for the parents of Jessica Ghawi and the Brady Center to Prevent Gun Violence.
It alleges it was illegal and negligent to sell the gear to James Holmes, who is accused of killing 12 people and injuring 70 in the 2012 attack.
Holmes pleaded not guilty by reason of insanity to charges of murder and attempted murder.
The suit says the companies had no safeguards to keep dangerous people from buying their goods.
"We think it’s unacceptable that a dangerous, mentally disturbed person can buy thousands of rounds of ammunition, body armor and tear gas at the click of a mouse without any questions asked or any screening whatsoever," says Jon Lowy of the Brady Center.
The lawsuit names the owners of bulkammo.com, bulletproofbodyarmorhq.com, BTP Arms and the Sportsman's Guide. None of the companies immediately returned telephone messages.