I have to admit that I've never approached ATCs and the like with a deliberate intention to use recycled materials. But in truth, most of the stuff I use for them is reclaimed materials. I'm a graphic designer for a printing company, so most of my cool papers are the trimmings from jobs we've printed; I've also claimed leftover parent size sheets (read: really, really big sheets) that would otherwise go in the trash. I've taken to dismantling paper swatch books, too, which would normally go in the trash once the new version comes out.
And I end up using lots of little bits of my leftover fancy papers from bookmaking, too. And all those images I can't bear to throw away...and all those supplies I never got around to using. In the end, ATCs (and inchies) have turned into an environmentally-responsible art form.