I'd have to say: I think this thread belongs in I LIKE HURR more than here. It really has more to do with people and their mannerisms than video games. The fact you work in a video game store is more incidental to the story than anything, this could've happened anywhere.
I worked in retail from the age of sixteen to roughly twenty-two, so I got a good six years of it. Across that time I'd worked at multiple retail spots (From Wal-Mart to Target to Circuit City Express to Best Buy to a cell phone accessory kiosk to part timing at another Circuit City WHILE working the cell phone place to a cell phone dealer for Nextel and T-Mobile). Honestly speaking, if one takes into account the sheer amount of people you deal with in retail, it's really not all that bad.
I can still remember my first "nightmare customer" at Wal-Mart. I'd been there like a week. On average, your worst customer at a Wal-Mart is going to be a middle aged guy. I was working in the electronics department and this guy comes moseying up (Oh yes, he entirely moseyed) with a carriage full of junk from other departments. Typically, we're supposed to send anyone with more than twelve items up to the front, but it was slow and I felt generous. Upon ringing him up, the guy starts berating me. Tells me I did it wrong and calls me incompetent and blah blah. I say fine and hand the guy a calculator. I tell him that if he's so certain MY REGISTER somehow added up his items wrong (I scanned down the totals and there were no duplicate items, so I didn't scan anything twice), he can add up the price tags himself and I'll adjust it appropriately to find my "mistake." He comes to the same total, starts bitching again and storms off.
First in a long line. I've been publicly berated, threatened with violence (Literally), SEEN coworkers threatened with violence, talked down to, called intimidating (This one actually drew a laugh from my boss), etc. I've dealt with countless situations mirroring each of the situations detailed in this thread (Through the majority of my job at the cell phone accessory dealer, I was covered in piercings and I've dealt with tons of hagglers).
Overall, though, in the grand scheme of things, even if you get one of these twats once every shift, that's still on average about one percent of all customers compared to 99% decent to really awesome customers. I've also now spent a good few years on the other side of the fence and noticed that a lot of retail workers really are condescending, patronizing twats most of the time (Guilty myself in retrospect) or completely incompetent. This isn't always the case, and when I deal with the former, I generally treat it with extreme praise. I had one occasion where I walked into a Circuit City with the intention of simply asking a question about cable converters and was sold on a digital camera that I absolutely loved. I hunted down the store manager, pointed out the clerk who assisted me and explained the details to the manager so the boss would know his employees were doing a phenomenal job. The thing to draw from this, though, is that the percentage of incompetent or rude clerks is a lot higher than the percentage of douche bag customers, in my experience from both sides.
I guess what I'm trying to say is: sack up, poor, misunderstood retail workers.