I'm a strange one - I have no interest in purchasing bills from others to add to my collection. I prefer to just see what I can get in circulation, the thrill of the hunt. With one special exception, all the bills in my collection are bills I have either found completely on my own, or in the case of most older bills I own, found by friendly bank tellers who saved them just for me.
I'm not out to make a profit, which also means I am also not a big stickler on condition - if I find a cool number but it has a crease, I'm still quite happy about it. So for me, I am doing a lot more than I used to, because I go through more quantity than I have in the past, but I know my experience is unique.