That’s a very low bar — denial of a constitutional right based on suspicion (albeit “appropriate”) about a person’s connections, and belief (albeit “reasonable” belief) about a person’s possible future actions. Indeed, most of the time this would come into play only as to people for whom the government doesn’t have proof of terrorist activity. If the government had proof, presumably the people would be prosecuted. (If the government has proof but isn’t prosecuting because it hopes that quietly watching them would help catch more or bigger fish, then barring gun purchases would be a bad idea, since that would alert the person to the government’s plans.)
I can’t see how that’s constitutional. And though the bill would have let the buyer go to court to challenge the attorney general’s decision, the attorney general would simply have had to show by a preponderance of the evidence that the two elements were satisfied — that the attorney general appropriately suspected the buyer and that she had a reasonable belief about what the buyer may do. Plus the evidence supporting the attorney general’s position might never be shared with the buyer, which may make it impossible for the buyer to fairly challenge it, or aired in open court.