I live in a university town with lots of rich kids, which seems like the kind of place to find good-quality used backpacks if there were one. If there's an easy way to get my hands on free used backpacks that I'd actually want to use, then I certainly don't know about it.
Also, when the semester gets out. Usually there is a [local college name] Christmas and the rich kids just put their stuff outside rather than transport it back home.
We used to raid the giant dumpsters they put in the parking lot and we'd always go to the dorm that had all the international students first because the loot was best. Scored an xbox 360 once, a monitor that still worked another time.
I live in a university town with lots of rich kids, which seems like the kind of place to find good-quality used backpacks if there were one. If there's an easy way to get my hands on free used backpacks that I'd actually want to use, then I certainly don't know about it.