I second that. In the UCC, a version of which Colorado has, there are warranties that are included in every sale of merchandise. But typically, I would suspect one of those warranties could be read to mean the merchandise isn't stolen.
That said, you should consider the implications of doing this, it could make it even more difficult for gun users to get guns because insurance companies raise premiums to defend yet another type of lawsuit that the store can't protect itself against--that is, if the store really can't check if it's stolen, they stop buying used guns or the insurance companies raise premiums.
The original poster wanted to have a law passed in CO to force the dealer to check.
Having said all that, you should consider that I recommended small claims court, not a class action lawsuit. If this store deals in so many stolen guns that their insurance would go up as a result of the avalanche of lawsuits, then that would be an economic incentive to check the background of the guns they buy.