Low Prices, you must be competitive, don't expect people to come into your store if your prices are higher than ebgames or gamestop. For example if ebgames is charging 8$ for a game used then don't charge more than that. If people see that your store is overpriced they will just hit forums like this one, ebay or your competitors for lower prices. I would not support an overpriced store, and if I come into the store once and see prices that are very out of whack (and I can buy the item on ebay for less) then I will likely never enter the store again.
Don't pressure customers into buying things they do not need like warranties and magazine subscriptions. This will probably not be an issue with an independent video game store. Also do not force games and accessories down your customers throat when they clearly don't want them. A simple, do you have any questions about this system or would you like any games or accessories with this system would be just fine if a customer is buying a system.
Price things based on condition, that old gba that is missing the battery cover and with a screen so heavily scratched you cannot even view the game, that some kid traded in is not worth more than 5$ to a customer. However a pristine condition system in the box should fetch a lot more than a junker system. Another area where gamestop fails, they do not price things based on condition, they just have a universal price for every item. So you might end up paying 59.99 for a scratched to heck GBA SP when you can get the same thing on ebay for about 30$ (I just bought a GBA SP on ebay for 33$ shipped with 3 games).
I would also suggest instituing a policy where you do not accept disks in horrible condition, or games in horrible condition. This is where gamestop fails completely, they will take anything you trade them and stick it right back on the shelf as soon as its traded in without ever doing anything to it. This is bad. If you can develop a reputation for carrying a good quality used product then you might be able to garner the customers that ebgames and gamestop has turned off. I would gladly pay a couple dollars extra for a complete and good condition copy of a game that includes the manual. There is a store in a fleamarket here that has been around for years and has a policy where they absolutely do not accept disks or games in horrible scratched condition, plus they have really low prices, I am now shopping there on a consistent basis instead of giving my money to gamestop for junk. If they can manage to do it so can you.
Presentable merchandise. We do not want filthy dirty, stickered up games thank you. We do not want nasty smelling dust covered systems. This stuff can be removed easily but its a pain to do, especially for your average customer that does not know too much about video games and how to take care of them. The store in the fleamarket knows how to do this, I see stuff that looks like it just came from my house there. Again this is an area where gamestop fails miserably with their stickered up and dirty games, often it takes me 30 min just to clean a game I get from there which translates to less play time for me.
Carry the stuff Gamestop DOESN'T. Gamestop and Gamecrazy have now pretty much discontinued the sale of older games, leaving some people with ebay as the only major alternative when finding an older game you want for your collection. A good selection of PS1, N64 and all older systems will bring people in, and you will get lots of trade in's from these systems. Tip : NES stuff is really hot right now, especially the most common games! This is another way you could step ahead of your competition, offer something that they do not. Gamestop has lost A LOT of business from me because they no longer carry stuff for past-gen systems.
Customer service, don't pester the customers, we do not want to be pestered. I agree about the scripted "hello's" thats so generic and very annoying. If it was slow instead you could have your employees ask a question like "what systems do you own", or another gaming related question. This would also help you gauge the customer, if you had 10 people come in, in one day that said they had an Xbox and wanted xbox games, then you could stock more of those. It will also create a more positive experience with the customers. When its busy do not try to sell a customer every product in the store... another thing gamestop does, when its busy instruct employees to move the lines along as quickly as possible.
Try to find employees that know something about games (not just employees that can push a product), this should not be too hard to do. Maybe let customers try out used games before they buy them, try to have some systems available and a TV so people can try out games. This way you can get people to believe in your product and you can assure them that you are not selling them a broken or non working game.