That is the whole point of the article. These guns were purchased illegally. It's not the purchaser that is at fault, but the seller that didn't follow the laws.
Private sales are legal in AZ, according to FReepers on other articles about this “sting”.
The sellers broke no laws and one of the other articles around here spells it out.
That is not actually in evidence as the article states Bloomberg enlisted the help of Rep Gifford's staff. There isn't evidence from the article that an out of state person purchased a handgun. Bloomberg has pulled this type of stunt before and got his hand slapped hard when he used out of state purchasers. Based on that and what was mentioned in the article I might surmise he attempted to use a state resident to do the purchase but focused on a non-dealer to make the case that the federal law provisions are inadequate.
If the "undercover" purchasers were not Arizona residents, or were not actually buying the firearms for themselves, then they as buyers were violating the law. IF they were actually Arizona residents, and they were buying the guns for themselves, then no laws were broken by either party. In a private sale nobody is required to do anything because one party says they "might" not pass a background check.
Of course lying about yourself when buying anything, as the purchasers apparently did, is pretty unsavory.