Be thankful you have a gaming store at all. The nearest one to me is 90 miles away so while I can see being upset about a little bit of rushing. I say, learn to live with it, and don't expect a guy you don't know to go out of his way to let you browse the store a little longer.
I have very little sympathy for people who bad mouth the local game store, I don't have one anymore because people who shared your mentality didn't give a few extra bucks a month to keeping one around.
Lucky this is Enworld and not somewhere else, man. I say deal with a little rough handling, maybe the man just had a bad day.
And your logic about him giving you the heads up that his store is closing is flawed. He told you he was closing, your lack of respect at getting what you needed, and getting gone was more of a rude gesture. He told you the information you needed, as a customer you should have respected his Time to Close, as what it was, instead of thinking he was just there to sale something to you.
I had the same deal happen when we still had the game store here in town, the guy behind the counter had somewhere to go, and told us four or five times closing would be happening in 10, 5, 0 minutes. Dude just didn't tell us he shut the store down turned off the lights and jangled his keys with a Warhammer 40k game mid process. Of course he turned the lights back on for the guys to gather their pieces but he didn't let them stick around for the final round. You want a rude retail guy he was it, and no one bothered to curse him out, they had been told it was their fault, they knew it. A lack of respect for others leads to this sort of thing. Just because he works at a store that garners little money doesn't give people the right to make him lag past his store hours. Those owners, employee's who do often do so because they don't have other things to do, or don't care about the hours posted. He obviously did. You don't want to give him your cash, don't whine about it, just don't do it.
Sorry to come off as a bit miffed about your post but, when you don't have something it is more than you can bear when others don't appreciate having what you lack.