Sometime around 1999-2000 or so I was at the Borden range in Indiana. When I left, I put all my equipment in the trunk of my car, and for whatever reason, I removed only the gun and left everything else, including the ammo. I did not shoot anywhere for a period of almost three years. I had left a couple of bricks of very good Lapua in the trunk of that car. Approximately three years later, back at the Borden range, I pulled that ammo out of the trunk to shoot. There was no detrimental effect whatsoever. It picked up right where it had left off as if it had been put in there yesterday. I do not reccomend storing ammo in such conditions. I have noticed a lot of people using ice chests to store ammo and most still try to keep it in the shade during a match. An ice chest, without ice, will get quite hot and humid on a hot, humid day. The best you can do, is to try to keep the ammo out of extreme conditions during a match. I think there are acute effects to temps, etc, but the ammo will settle back to it's original state at least most of the time.
As far as buying ammo stored in the garage. It probably was not a prized lot and, therefore, will probably not be great ammo. Most likely, it will go "pop" everytime and may be as good as average lower grade ammo. I guess it depends on the price. Shouldn't be more than around $3-4 a box to make it interesting.