Federal authorities had accused the store earlier this month of selling 10 guns to people who employees "knew or had reason to believe" were illegal straw buyers.
Activists have picketed Colosimo's Gun Center because of the number of guns sold there that end up being used in crimes. Colosimo has said that's to be expected from a high-volume dealership like his and that he follows all laws.
The plea agreement calls for the store to close on Wednesday. Sentencing is scheduled for Jan. 6.