I've seen this at a local shop I sometimes go to and have purchased from before (Shore Galleries in the Chicagoland area... I recommend them for small stock but great prices!). Reading the posts about this issue cleared up a lot of questions I had as well! I think, however, that here it's because there is so much patchwork in the law that selling an AR-15 in Cook County is just too risky unless they are LEO. All that aside...
I fully understand that he couldn't sell it to you (it was marked as such), for whatever reason, and that must be respected. What doesn't settle with me is that he had no reason to give and was unwilling to do more research into the fact. He didn't even offer to order one for you? For what it's worth, if he is unwilling to take your money, take the hint. I'm seeing the exact same gun you want on Gunbroker for around the same price and even less. Can you imagine the look on his manager's face when he's told about his idiot employee let $700 walk out the door? I don't expect all people to understand sales (as I am in the profession), but come on? Are you mental?!? You would've walked out of the store without the money if I were the boss!
"Shut up, crime!"