There's one out in Brighton. Not worth visiting though. They have a good selection but the prices are outrageous. For a new copy of Gun Valkyrie they want $40 last I checked. If you're interested and willing to make the drive then go ahead. They do have a decent selection. The place is called Game Trader Inc.
There's also the Franchise store 'Play N Trade' out in Arvada off of Kipling. I know they sale vintage stuff. Prices will vary. Not that great a selection though. Apparently there's one in Longmont as well. The one in Aurora closed down a while back (pretty sure the last time I was there, closed with nothing inside). Don't know if all of them sale older stuff but I know the one in Arvada does.
A few weeks back I was down at Colorado Mills Mall. Didn't realize that inside there's another video game store besides Gamestop. I believe it was called 'AZ Games'. I know they sale older games but wasn't too keen on the prices. Selection isn't great, too many multiples of certain titles, and I personally thought it was very unorganized since many of the titles weren't in alphabetical order when I was there.
Let's see....that's all the ones I know of that haven't been discussed yet.
Also, anyone happen to know if the Game Force in Boulder is still open?! That is probably the only store I've really ever been impressed by due to selection since they carried all sorts of import titles.