They could delay purchases 3 days now technically by simply refusing to give the okay on the NICS check. What they would try to do in 3 days is another matter.
I have a feeling the FedGov wouldn't want to have to go to court to deny the sale as that would require them to prove their suspicions in court. If they could do that, it wouldn't just be a "watch" list. I figure they have little or no evidence on most all the people on the current no-fly list. I doubt a no-gun list would be any better.
Also, would a no-buy list just deny sales or would they try to come take away guns currently owned.